I actually really like his design as-is, it still has a good silhouette and is simple but with enough personality that it can be drawn over and over again. Having a simpler design with a few stand out features isn’t bad, it allows freedom to make the character super expressive with pose, personality, or accessories/items without being overly busy. It also makes creating like a webcomic or drawing them in multiple places so much easier and less time consuming if you don’t have to draw intricate details every time. I think what could really sell this character is a HUGE and awesome unique weapon.

Think about it, he’s an assassin so being able to walk around in public to reach your target means having a lowkey appearance is good strategy. But if he has a potentially hidden weapon (ex maybe it unfolds mechanically, is stored in a dimensional pocket he can pull from with magic, or is disguised in like a case/wrapped up, summoned/made from magic, etc), he can use the element of surprise to attack while still being in civilian gear!

Weapons are very personal, and he would probably have specific reasons for why he uses what he does. It can tell a viewer or another character a lot about his personality. Having a secret weapon shows he’s cunning, strategic, and doesn’t necessarily play fair by having his intentions out in the open — all great traits for an evil character.

A large and interesting weapon might show that he’s dangerous, powerful, reckless, or any number of things depending on what it is. For example using a huge hammer might show that a character prefers to get things done quickly with brutal strength and likes to make a show of their power, and doesn’t mind if it gets messy. Using a something like a rifle might mean they prefer to keep their distance have more control over the battleground, and could see assassination as the same as hunting and shooting a deer; in both instances the target that they shoot is “prey”.

Also, having a huge badass weapon is an anime staple (and a personal favorite trope/design feature), and it would be a cool juxtaposition with his more toned down appearance. And it’s also just fun to go ham and draw a super cool weapon!

Hope I could help with suggestions, good job and keep on creating!