1) Why does Roxas resemble Ventus?
2) Ansem is Xehanorts nobody, right? So does that make Xemnas a nobody of a nobody?
1) When Ventus's broken heart hid inside Sora (thus giving Sora the ability to wield the keyblade), their hearts effectively became one in the same. Taking this into account, when Sora unlocked his heart with Riku's keyblade and became a heartless, his nobody took on the form of Ventus, thus Roxas. If his heartless was sufficiently strong enough and could resemble a human (like Xehanort's heartless Ansem) it is entirely possible that the heartless would resemble Ventus also.
2) Ansem seeker of darkness is the -heartless- of Xehanort, Xemnas is simply the nobody of Xehanort.