Avenger Shinjuku

Give him stars, someone like Hans or Helena would work.
Has a buff removal on his 3rd with a 5 turn cd so that's useful.

Other than that he has an unreliable instakill on his NP for a single target.

personally, for how cool he look, i do not think he is worth investing in him now. having an avenger can alweys be usefull, but invest on more usefull servant for now.
For what i can see, he is a quick servant, with no star absorbtion (you need the firstskill maxed to win over rider, and for 1 turn only) with low star generetion, so you need something to create star.
He also lack any hard survival skill, and his only def last 1 turn and is tied to his best offensive skill, any good support for a quick servant, and, for me, his best skill is the third one, (attack down, and buff remuval), and is the one you should max last.

unless you really like the servant, i would advise to wait before enanching him. eventually he can be usefull.
at least this is what i would do, but i have many servant who can do his role better (edmond, cu alter, herc....ecc ecc)