Ooohhh this is kinda a tough one....

While the most obvious trait would be his Courage (King of Braves and all that), one that I love is his dedication: while it's mostly shown as a dedication to protect people, it all started as his dedication towards his dream: to be an astronaut and reach Jupiter and beyond, which was very nice to see in a hot-blooded protag that wasn't always just "I was kinda aimless before the events of the story"

Another big one is his brotherly bond with Mamoru.... Idk it's nice to see him be an older brother figure to the young kid, which let's his goofier moments shine + in GGG v B, when they get all drunk and sappy together talking about their lovers (makes him forget he can sober up instantly as an Evoluder lol)

Also I'm just a sucker for a long haired kinda bishonen dude, but that's another thing entirely lol