"Sincere" is a complete word on its own, meaning "free from pretense or hypocrisy; genuine".
However, you could use it as a base for other words:
* Sincerely: This is an adverb meaning "in a sincere manner."
* Sincerity: This is a noun meaning "the quality of being sincere."
* Insincerity: This is a noun meaning "the state of not being sincere."
Let me know if you have other questions!