The First Descendant

This guide is for players who don't intend to purchase the Descendants from the in-game shop.
First Descendant is what you start with in this game. And I would recommend starting with Viessa because she is the only Descendant with a Chill Attribute type. You can start with Lepic and Ajax too but there are other Descendants with the same attribute type. This will help in completing Void Fusion Reactor missions (later in the game and will come in handy for this guide)
Second Descendant will be Bunny as you get her parts as part of story progression (& tutorials) with a minor hiccup. During your first Void Intercept Battle with Grave Walker (basically a full version of the one you got in the Prologue), make sure to select the third Amorphous material (it should be labeled as "Bunny") when you interact with the floating sphere post defeating the boss, here in this case, the Grave Walker.

To get full details of any Descendant you intend to unlock, simply select the view button on your controller and select "Access Info" > "Descendant". Apart from Bunny there are 3 Descendants that you can unlock without needing to research individual parts. They are Freyna, Sharen and Blair.

Lets start with Freyna. Because she is the earliest of the three to unlock in this list. Once you select her on the Descendant page, you will see 4 parts that are required. Hover over each part and press X to get the mission(s) you need to complete in order to get that part. However, there is a catch. There is only a 20% chance from the Special Operation missions and 38% chance with Void Fusion Reactor missions (for which you need to complete Void Fragment missions multiple times and get the corresponding Amorphous material). As I said, it comes down to luck. I personally got lucky as I got her parts in 1st & 2nd tries.

Now instead of grinding missions for Sharen when you reach Sterile Land & Echo Swamp, I would highly recommend you to first complete the story with the 3 Descendants and rank them up as you go.
Because in this game, enemies do not scale with you. They scale with the region. So once you hit level 100, you can go back to the areas and wiping out enemies a lot faster. And making your grind very easy. You can ignore Freyna too but you might have a hard time in Boss Battles that are resistant to both Chill and Electric types. So having a 3rd Descendant always helps (they should be level 30+ too so best level up Descendants parallely).

Another thing I would like to add is that you can access all the Special Operation missions from Albion by going to the (o) icon on the map instead of going to that specific zone and running over there. Its because every time you complete any Boss battle or Special Operation, game will drop you off at Albion. So its easier to replay the missions.

Note: I personally couldn't get a single Sharen part after running her missions 5-6 times each and I had couple parts for Valby already so I just pursued her and was honestly easier than unlocking Blair lol. Worked out for me as now I got Descendants of all Attribute types including Valby who is Non-Attribute type. Feel free to share if I missed anything or got any piece of information wrong.