Tlok//spoilers

Mako did have character development.
Yes, Mako's role in the show is the cool, hot love interest, which isn't objectively a bad thing. I mean sure, a redemption or a revenge arc would've made him more interesting, but the lack there of isn't inherently bad..

He starts off in s1 and s2 as a literal piece of shit towards both Asami and Korra. From cheating to lying, this guy was simply intolrable. He was a great bender and very reliable as a teammate, but certainly not as a boyfriend..

In s3 he realises how fucked his past with Korra and Asami was, so he decides to try to stay away from them completely and to treat them professionally and coldly because he believed things will never be normal, especially now that Korrasami are getting "friendly".

Lastly, in s4 he grows up and moves on and focuses on what really matters, which is being behind Korra whenever she needs him as a friend and as an ally. He doesn't need to be a "boyfriend"; he's a guy whom his closest friend, the avatar, can rely on no matter what.

All I'm saying is tlok is such a great show. Is it flawless or perfect? NO.
Just try to cut it some slack, & stop nitpicking on every character for not having to go on a life changing journey just like atla characters had to. There is honestly much to appreciate about the show!

Anyways, let me go continue rewatching the show knowing that no one will actually read this thread.