が一人います

there is one …; have one …

→ が
phrase Rare #22,735
there is one …; have one … Used after a person or family-member word to say that one person exists or that someone has one of them. The best English translation depends on context.
JA: 友達が一人います。
EN: I have one friend.

私には留学している友達が一人います。

I have a friend who is studying abroad.

子供が一人います。

I have one child.

トムには弟が一人います。

Tom has a younger brother.

教室に背の高い男の人が一人います。

There is a tall man in the classroom.

兄が一人、妹が一人います。

I have one older brother and one younger sister.

息子が一人います。

I have one son.