Ritsu Sohma is male, though, like several other of the Sohma guys, has a slight female-like appearance. He is cursed with being the monkey spirit in the zodiac. He can be seen dressing like a girl at times as it calms him down by easing his nerves and can help him feel better about himself.
Ritsu, like his mother Okami Sohma, the hot springs lady/hostess, is extremely apologetic in nature and can often be found apologizing for his actions in a dramatic and overexagerated manner along with apologizing for things that weren't exactly his fault. However, he is also very sweet and kind. He has low self esteem but is a beautiful person in general. Ritsu is very unstable, as stated with his apologizing, and overreacts to everything. Momiji once said that he has a kind heart.
As a child Ritsu was always seeing his parents apologize for what he did and how he was. This was what spawned his insecurity and what started his cross-dressing. Dressing in girls' clothing made him feel good about himself, but this act only made his parents apologize even more.