Knockout City

Player icons/Face/hairstyle/gloves/ultimate hologram all definitively count towards this achievement

Edit: outfit also counts

tips to unlock some of these items:
1. Keep leveling up and doing challenges

-leveling up your street rank offers items that have the potential to be legendary. you also get energy drinks which will help you level up more and you can get holobucks to buy items

2. Check the store each day

-the store refreshes daily and has the potential for legendary items to drop. if you hoving over the item it'll say what the rarity is

edit2: legendary items can be fairly expensive. I've only seen a legendary outfit in the store twice at the time of writing this edit (6/19/21) and It costed 1,200 holobucks both times.

800 for legendary glasses

3. Redeem the Pride player icons

- hit start, go down to 'more', choose to redeem code, and enter 52T3-BMJD-3QNL-BQWB-LWK9
- This unlocks all of the Pride icons (21 in total), all of which are considered legendary. The devs claim these can be redeemed forever