Bungo Stray Dogs maybe?
also followingヾ(❀╹◡╹)ﾉ~
Boku no hero Academia
Sedlyf is when 'Breaker' could've been so much better but it just turned out to be a bunch of kids tryna act macho...
dokgo rewind (its a sequel tho)
Berserk, it may not have the weak to strong transition you're looking for (Guts is always a beast) but the character development on a mental and emotional level is amazing. Plus, while he's strong, he always goes up against overpowered enemies, so it kind of balances it out. One of my favorites.
Also try out Houseki no Kuni, I just caught up with it yesterday. The main character is born as the weakest of the bunch but becomes one of the strongest by the latest chapter. I really really really love it so far, probably one of my favorites as well.
Akatsuki no yona has the best character development!(⌒▽⌒)
You should try it! It has an anime adaptation too!
IF U WANT CHARACTER DEVELOPMENT THEN U SHOULD MOST DEFINITELY TRY "LOOKISM" YOU WONT REGRET IT
I'm looking for a manga with character development or where a character improves themselves from "weak" to "strong" doesn't necessariliy have to be a strength either could be improving their situation in life like from "bum to boss". So far manga that kinda have that feel to me are Kingdom, the breaker and the breaker new waves, The One. Can anyone recommend some for me please. Thanks!