Quick assist guide

Assists are affected in two ways: LB level and affection gauge
You can only equip assists if the y are level 70
There are two things that happen with it: one you get a button that gives buffs, 99% of the time its the characters rush buff and the other is a boost to your rush damage

LB levels
Determines the boost to your rush gauge, lb 1 will give +10% while mlb will give 100%
The button buff also has two levels, 1 and 2. Only MLB will give you Lv 2 rush buff
Lv 1 buff is a weaker buff than they normally give, about 10-20% weaker
eg. LB6 Caleen will give 20% crit chance+damage while LB 10 will give 30% when you press the button

Affection gauge
Determines the chance of the rush assist procing
Max affection gives you +1 ap
You can check the characters overall affection level by long pressing on the character, there will be hearts on the top right of the portrait of the character. It goes from 1-4 hearts
Collab characters dont get affection

You also get gifts for maxed affection characters, mostly seed, can possibly get ap seeds, grease, carrots lbs, etc
You will get free stam once a day too determined by how many characters are doki doki for you
You can claim it any time of the day, it will be on your bridge on the top left
It doesnt carry over to the next day

Note: Character alts count as the same person
eg. maxing out beach Tika will also max out normal Tika, Kimono Tika and Titka

https://wikiwiki.jp/soanamnesis/%E3%82%A2%E3%82%B7%E3%82%B9%E3%83%88%E3%82%B9%E3%82%AD%E3%83%AB

Notable rush assists
Abel - 100% atk -50% def (self)
Eve - 100% int (self)
'Kay - 100% int, 70% cast speed (10s) (self)
Henri / SF Fatty - 80% atk -30% ap(self)
NY Tika / valka - ST dmg 50%
Ricardo -  80% atk 50% crit rate (self)
Mastema - Crit rate+Dmg 50%
Emmerson / Summer roddick - 80% atk (self)
Summer Edge - 80% atk 15%/3s hp regen (self)
Arche - 80* elem dmg
Idol eve - 70% elem dmg
Nel - poison on normals (self)
XXXclaire - atk/int dmg +50% 30% hp heal
Sol - -80% ap (10s) (self)
Hat Nel - 60% atk dmg
Idol fatty - 80% atk 10%/3 hp regen (self)
Claire - 80% dmg
Adray - 100% dmg(self), 80% dmg (team)
Beach fox / elephant - -50% ap

I wouldn't bother with elem dmg assists unless its arche or idol eve, modern weapon have ~40% elem damage built into them already