I'm wondering if yōsei means someone's name or "fairly tail", but it's not a main topic in thi situation^^ yōsei no tomo... correct yōsei no tomodachi...correct yōsei no yūjin...correct yōsei yūjin...wrong When you say "a friend of someone", you can say normaly " someone no tomodachi " in Japanese. For example, "I'm a friend of Tom" means "私はトムさんの友達です　（Watashi ha Tom-san no tomodachi desu）” "tomodachi" is the most common word which means friend. "tomo" and "yujin" also mean friend. And I think "yujin" is more formal than "Tomodachi". If I introduce myself to elder people, I'll say "Watashi ha Tom-san no yujin desu". About "tomo", we rarely use this word as a meaning of friend. Hope this will help ;)

You can use "tomodachi" in any case. "yujin" can be used for the relationship over teenage. I cant understand what "yosei" means exactly. When it means "positive" or "good", "yoi tomodachi" is most useful.