Did you know that some Russian nouns can be both masculine and feminine? They end in а or я and are declined as if they were feminine, but the adjectives, pronouns, participles and verbs used with them can be masculine and feminine, depending on the situation. For example, you can say приятный коллега (“a nice colleague”, masculine) and приятная коллега (“a nice colleague”, feminine).
Some more examples:
Грязнуля: unclean person
Жадина: greedy person
Зануда: boring person
Левша: left-handed person, etc.
You can find the whole list here.