Is Leonidas Valdez the perfect character? Before I begin,please take this with a grain of salt as I haven't read these books in a year and I have a somewhat biased opinion. Now,let me begin.
Leo is a dynamic character with an interesting past. His mother died when he was 8 years old (who he was quite bonded with),and he never knew who his father even was until he was 15. He suffered a rough foster care life,had delinquent like habits,and would often be in trouble with the law for his run away tendencies. When Leo meets Hephaestus for the first time,he argues with him about the past. By ToN,Leo doesn't necessarily HATE him anymore and treats him like a dad. Leo also experiences general development. At first,he was a hopeless romantic teen who relied on other people for his happiness. While you can make the argument he hasn't really changed,I think he has made SOME development. I will agree,though,he is still weening off other people to support him. Like with Calypso(and every person he's ever romantically liked) ,how he feels about himself reflects off about how they feel about him. When it was revealed that she actually did reciprocate feelings,Leo felt loved for once. He never loved himself,and probably still doesn't. But then why is he the "perfect character?" I consider him this as he goes through some development,but not so drastic within a short period of time that it becomes unrealistic and makes him flawless. People change,but sometimes it isn't always for the better. (Leo's change is semi-controversial,so I am not going to label it as good or bad)
Beyond the books
The fandom loves Leo,and for good reasons. His character is literally timeless. Despite the books being written over one decade ago,they are still popular among middle grade aged kids and teens today. Not only that,but you still love his character even years and years later. With many people (including myself),you initially feel attracted to his character due to his physique,"edgy" attributes,and "deep" backstory. But later on,you start to appreciate his inner complexity. I still love his character to this day,and so do many other people like myself. Beyond development,I think his overall character is a whole subject to debate. Some argue that he isn't fit for being a traditional protagonist,and he's more of just a comedic relief,antihero,or would work well as an antagonist. Is he a good or bad character? That's up for you and only you to decide. That's what I love (but also hate with all my heart)about literature,it's all up to interpretation. Your opinion is always the fact in your world,and nothing can stop that.