In complete seriousness: you need to

  A) give him a catchphrase

  B) have someone use him for their own purposes and turn out to be a friend later on after saving him from a tough spot

  C) make sure he has no luck with girls except herione and make her tsundere later on.

  This story originates from a different website. Ensure the author gets the support they deserve by reading it there.

  D) add more random fillers to ensure suspense and it also gives you some relaxing time+opportunities to add more characters.

  E)amnesia arc: a slime man with amnesia, you gotta love it.