"Matěj, you are a good friend."
Translation:Matěji, vy jste dobrý přítel.
It is OK in this case. The word "přítel[kyně]" implies a closer and more trustful relationship than "kamarád[ka]". The sentence sounds very earnest, like something you would say after your friend would offer you help in a difficult time.
When speaking about boyfriends and girlfriends, possessive pronouns are usually present ("můj přítel", "moje přítelkyně").