this au is called Not so bland memories au and is a ninjago au NSBM au was another version of it call bland memories.
BM : kai never becomes a ninja. he still goes with wu to train and right after he saves nya he leaves the ninjas and nya follows with him. in this short time the others swear they have seen him before but kai shakes it off. Tho in the time they met the others fall for kai and kai finds out, to relive them of the pain when he leaves he gives them a special memory erasing tea that ereases the memories of the one who hands them the tea the night before he leaves. fast forward around 10 years (like s8 time area)) kai has become a chef and part time trainer in nyas dojo when a confused and yet terrified cole gets thrown right through the wall of kais resturant only to instantly feel like he has seen him before. kai panics and hides his face before cole could get a good loke of his face and remember how kai looks.
NSBM :kai actually stayed to until lloyd became the green ninja and then left without nya to become a chef in the outskirts of ninjago city and instead of giving them memory ereasing tea h drinks it himself to forget the others (including nya) in this au the ninjas meet kai again, when hes walking out of his resturant and the ninjas have somehow found him and tries to remind him but he just is annoyed at the ninjas bc he sees them as attention seeking loud and self centered heros. one day gamardon(who kai knows jack shit about because he lives under a rock) finds him and first was planning on making him his second in comman but tries kais food and decides kai shall be his 'evil' chef and kai agrees because well uh, hes nice and he pays pretty well this on the other hand beaks the ninjas hearts and their convinced gamardon cursed kai or some shit