Looks:
Crona has, and always had, a rather effeminate appearance, due to which it is often unclear to people that meet him as to what gender he is. He is, however much he might look like he's not, a guy underneath his long, all covering black outfit. Crona wears this same outfit, as seen in the picture above, almost all the time. The only time he might wear something else is when someone forces him into it. Kind of like a friend making him try out new things. All and all Crona is rather scrawny, a lanky, skinny boy who doesn't look very strong.
Background:
Crona was raised from a young age by his mother, the witch Medusa, to be the best killing machine possible. She actively tried to turn him into a monster, injecting him with the demon sword Ragnarok and forcing him to kill with negative reinforcement. His mother never pushed him far enough to make him become totally ruthless however, as Crona had natural inclinations for good that resisted totally succumbing to becoming a monster.
Personality:
When one meets Crona the most striking feature is that he is extremely nervous and shy. Crona has a extremely crippled self esteem that is mostly due to a harsh life in the past. This said he is rather open to people wanting to be friend with him, though sadly he's too nervous to be proactive in meeting new people. For people that do become friends with him he is a rather sweet individual. He cares about his friends a lot, as his experiences without them help him cherish the new bonds. However...
Insanity:
Crona's past did not leave him unscathed, he was left with the scar of an unstable mind. Acts done in his past has caused his mental state to be somewhat questionable. Most times Crona is kind and caring, but with ample trigger he can become a ruthless monster, killing with insatiable desire for violence. In this state no one, not even his closest friends are guaranteed to be safe.
Ragnarok (Demon Sword) & fighting style:
This is the weapon you see Crona wielding in the first picture. Crona's weapon is in fact made of his own blood. Crona has black blood that was injected into his body at a young age. This blood can flow normally like liquid or go solid, and is completely at his control. That means cutting into his body is near impossible, as his blood can stop the blade from going deep inside. It also means that in instances when his blood is spilled, it can be used as a weapon, forming spikes from puddles of it, or launching beads of blood like sharp needles. This isn't all his blood can do, you see, Ragnarok is more than just a sword. The creature with Crona is also Ragnarok. It can form out of the boy's back and morph into different forms, from swords, to wings, to it's burly more humanoid appearance.