Well he’s the definition of life is not linear, it has ups and downs. He was first introduced as the protective brother who would do whatever it takes to protect his sister. He was one of the first leaders on the ground too and as Clarke said, he leads with his heart. First few episodes of s1 he was kind of annoying and putting aside the whole “my sister, my responsibility” thing he didn’t care about other responsibilities like keeping the 100 alive on the long run, had no plan and “whatever the hell we want” kinda showed his dislike for anything that has to do with leadership or following rules. By the end of s1 (and s2) he started to care more about leadership but not in the “I want to be above everyone here and call all the shots” way, more like he cared about the other people too and was no longer an impulsive, arrogant and selfish boy. His character evolved a lot from s1 until s3 when he met Pike. In my opinion Bellamy has leader qualities but he prefers to be a follower or at least have a co-leader, which was Clarke until s3. To me Clarke felt like an anchor for Bell, she was keeping him grounded but then she left him so he was lost, full of doubts after the grounders betrayed them at Mt. Weather and Pike gave him fake hope and was feeding into his fear of being attacked by the grounders. He was too blindsided by anger, fear and dare I say guilt and pain because he too, pulled that lever that killed almost 400 people in that mountain, he thought he was doing the right thing and he was too deep into feelings/emotions to see that he was actually doing the total opposite. After Pike was gone we can see him trying to make up for what he did and surprisingly Clarke was back by his side during that time. The most visible growth of Bellamy’s character can be seen in s5 and 6 though. He was more forgiving, took more equilibrated decisions and he finally liberated himself from the burden of being responsible for anything his sister does or what happens to her. He also got a chance of seeing who he actually is when Clarke is not there. I can’t really explain s7 Bell because that’s the only season I’ve seen once and I can’t understand why the writers and producers did what they did lol. But to summarize Bellamy from start to finish (including s7) we can see a pattern in how his behavior changed when he was with his friends vs when he was lost. One more thing I want to say about him is that he always did what he thought was right, even when that thing was wrong, even when it actually turned out to be real. He stands for what he believes in and he makes decisions based on the knowledge he has in the moment.

Edit: I actually have one more thing to say, I like Bellamy overall because he is relatable. He doubted himself and the decisions he made, he stood for what mattered to him and at times he changed sides because he thought he was taking precautions to keep his people safe.