The Witcher 3: Wild Hunt

This trophy takes a bit of pre-planning from pretty much the very moment you enter Velen(aka No Man's Land). If you've progressed past a point of a important decision, there's no returning unless you just happen to have a save there.

There'll be some hefty spoilers ahead, but for the sake of clarity(especially in Keira Metz's case) I'll have to mention them, since some of the folks that you can get as allies have quest lines with some incredibly hard to detect outcomes.

And one thing before we set off here, the trophy pops as soon as you finish speaking with the last person in your Brothers in Arms quest, some of the people here will be determined to go to Kaer Morhen before this quest takes place so they won't need to be talked to.

For the sake of completion, I'm listing all possible allies even though you need only 7: Keira Metz, Vernon Roche & Ves, Triss Merigold, Zoltan Chivay, Ermion and Hjalmar an Craite.

KEIRA METZ - During Act I in Velen - #1
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In order to get Keira on your team, you need to do her entire quest line in Velen's bogs. The quests An Invitation from Keira Metz and A Favor for a Friend are a given in her chain(whether or not you sleep with her isn't important), the important bit comes in at the end of For the Advancement of Learning:

She'll come up with the idea to present what she's found to Radovid in Novigrad, you'll get three options to answer her with:
Tell her its suicide, she'll give the more feisty reply and begin getting upset about the state of her living accommodations, tell her "Go to Kaer Morhen" and finish with "See you at Kaer Morhen", simple as that. She sets off and you'll spot her later post Isle of Mists in Kaer Morhen.

If you let her go to Radovid, you'll find her later in the story during Novigrad: Closed City burning at a stake. And the more violent sounding option of "can't let you do that" leads to combat with Keira, so obviously this option will not get her support as well... as she's dead.

LETHO - During Act I in Velen - Only if Letho lives in your save.
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So this one is tricky, Letho only counts if he lives in your save state. If you chose "I let him live" in regards to Letho during the shave-n'-cut in Vizima to Morvran Voorhis during Imperial Audience, the quest Ghosts of the Past becomes available.

The quest's climax is again where the choices matter; make sure you either interfere Vester and kick his ass or let him and his buddies leave without taking Letho's head but his amulet instead.

Interfering is the safer option in the end and gives some lewts, anyway after the guy is dealt with, Letho reveals himself to be quite alive, you can now ask him if he wants to be welcomed back to Kaer Morhen, do so.

EMHYR VAR EMREIS - During Act II in Vizima
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Easy one, just pop by the Vizima castle during Brothers in Arms: Nilfgaard and speak to good ol' King Deathstare Grumpypants. He'll be the usual wrench in the works in terms of actually giving help, but in the end he pulls through and counts as an ally.

VERNON ROCHE & VES - During Act I in Novigrad - #2 and 3
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You'll be referenced to these two through your search for Ciri in Novigrad a few times and after doing Roche's quest for meeting King Radovid V, he'll have a quest available in which you go after Ves, who's run off to save a village from Nilfgaardians, This quest, An Eye for an Eye has to be completed in order for them to be eligible during Brothers in Arms: Novigrad. Speak to him during that quest and he'll happily join you at Kaer Morhen along with his partner in war, Ves.

Thanks to jason_ex for the correction that whether you kill or let the Nilfgaardian survivor go doesn't affect Ves' willingness to join Roche.

ZOLTAN CHIVAY - During Act I in Novigrad - #4
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Zoltan is always ready to fight by Geralts side, just speaking with him at the Rosemary and Thyme/Chameleon makes him an ally.

SIGISMUND "SIGI" DIJKSTRA - During Act I in Novigrad
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Another Keira Metz situation, you need to fulfill very specific conditions for him to offer help. The choices you make during the quest Count Reuven's Treasure are the ones that make the difference, specifically during the segment where you deliver Triss Merigold to Caleb Menge and his Witch Hunters. You can ruin this by just having a sharp tongue, so be sure to follow the instructions:

1: Upon getting to the Witch Hunters Stronghold, you'll be greeted by a Witch Hunter, keep your cool and basically be cold to them about Triss. They say they're gonna rip her fingernails off, just agree and let them. Saying there won't be any ripping leads to a big fight.

2: When following the hunter, stay right on him, don't loot anything or deviate from his trail, they'll take your curiousity as a hostile nosy act and your infiltration plan fails.

3: At the top of the stairs the torturer will be taking Triss, answer "Fine, take her" and it goes on as usual.

4: You'll now be seated with Caleb Menge, during the following chat you'll have to answer with these:

Menge makes you drink from a silver goblet to make sure you're not a doppler, say "Why not."

Kurt's being told to leave the room and Menge further questions you about your wits and whatnot while Triss' torturing begins, when the dialogue choices come up say:
"Your hunters are torturing Triss, but I haven't been paid."

He then goes on about your motivations in regards to turning Triss in to start with, either one of the answers works:
"Triss betrayed me first." or "Witchers do anything for gold."

He'll be praising your gutsy attitude soon after and bring up Phillipa Eilhart, at the last dialogue choice here say:
"Free my friend Dandelion."

5: Now **** hits the fan, you and Menge go to investigate the torture room as it went quiet, Triss got free and gains the upper hand on Menge, stabbing him clean through the chin, killing him. Talk to Triss, take the key and walk back to Menge's office. Sneak out through the balcony door at the back of the room and just carry on with the quest.

That's the whole "subterfuge" way that Dijkstra demands for all of this to be good and dandy. During Brothers in Arms: Novigrad, he won't actually be of much help but still won't make you leave empty-handed.

TRISS MERIGOLD - During Act I in Novigrad - #5
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The quest Now or Never has to be started and done for her to go to Kaer Morhen, and you gotta be nice to her about it too. Be supportive in dialogue choices for most the start of the quest, such as answering I will when being asked if you'll help right now.

The choice that matters is again at the end of the quest:
Triss is adamant about going with the mages on the ship, you need to tell her "Stay with me" when the opportunity arises.

She'll say she can't, and won't do it. Now you'll get three options, choose "I love you." She still goes, after a bit of a cheer up from Sigi Dijkstra, Triss will suddenly be back. Make sure you answer nice and politely, saying you like her changeability won't get you anywhere.

You'll go look at the boats from the lighthouse and do a bit of motherboating of your own. This prompts Triss to stay in Novigrad and automatically make her way to Kaer Morhen during Brothers in Arms: Novigrad.

EDIT: As of update 1.02:
Triss will be invited to Kaer Mohren during Brothers in Arms by Yennifer if you let her go during Now or Never and shows up anyway, hurray for no forced love!

CRACH AN CRAITE - During Act I in Skellige
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Meeting with Crach is fruitless in terms of manpower, but he does give you a nice steel blade of legendary status called "Winter's Blade".

CERYS AN CRAITE - During Act I in Skellige
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Complete the quests "Possession" where you free Udalryk's mind of a Hym, "King's Gambit"(pick looking for clues in the hall with Cerys to ensure her becoming queen) and "Coronation".

It's very important that you retrieve enough evidence to support the an Craite clan against Lady Bran, eventually her son will testify against her and she'll be outcast. If you decide to not help either Cerys or Hjalmar, Svanrige Bran will be crowned King and you will not have any allies from Skellige's royal house for Kaer Morhen.

After her crowning coronation, speak to her in Kaer Trolde, she'll say her duties prevent her from helping personally but her brother Hjalmar sure as hell will be up for the bloodshed, see Hjalmar's entry below for more information.

HJALMAR AN CRAITE - During Act I in Skellige - #6
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Complete the quest "The Lord of Undvik", and then the path will merge onto Cerys' path. You can choose to help her instead of Hjalmar or go with Hjalmar on his blood craving revenge quest during "King's Gambit", either way whoever ends up being crowned during "Coronation" isn't very important in regards to Hjalmar's status as an ally, he'll always be up for kicking some ass in your name.

Just go and chat him up in Kaer Trolde during Brothers in Arms and ask for his help.

FOLAN & VIGI - During Act I in Skellige
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These two are optional but still nice to have, they're both part of Hjalmar's hunting party that got wrecked and marooned on Undvik with him during "The Lord of Undvik". During the quest, first head to the Abandoned Village west from Marlin's Coast and find Folan in the troll cave a bit north. He's currently being boiled alive so make a manual save before instigating conversation with the ice trolls.

They strike a deal with you, beat their riddle with your wits and Folan is yours. You got ten seconds to answer the riddle or he boils alive, the trolls still give you a second chance for a riddle if you mess up the answer on the first one but Folan will still boil alive, sadly.

Riddle #1: "A troll".
Second riddle? Well that doesn't matter, Folan'll be boiling like a egg by this point.

In regards to Vigi, it's quite simple. Just progress to the end of the quest and you'll find him in a cage near the mountain giant. Say you'll free him and you'll be allowed to walk around to find the key.

The key is in a chest near the giant, be sure not to walk on any of the ice patches or sprint in general so the big guy doesn't wake up. Loot the key, set Vigi free and he'll go running at the giant like a maniac but at least he's alive.

Failed to get the key without waking the giant up? Well, bad news. He'll crush and kill Vigi during the second stage of the fight where he rips the anchor from the makeshift prison Vigi's in.

Once you rescue both Folan and Vigi, they'll automatically come with Hjalmar during Brothers in Arms: Skellige.

ERMION - During Act I in Skellige - #7
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Easiest for last, while you're chatting to Cerys and/or Hjalmar, pop by Ermion(he's either in Kaer Trolde with the rest or at the druids over at Gedyneith) and ask for his help. He immediately says yes and makes way for Kaer Morhen during Brothers in Arms: Skellige.


And that's all, the required seven plus all the optional ones. As I said, the trophy pops once you chat to the last person who'll agree to go to Kael Morhen.