が一人います

there is one person

→ が
phrase Rare #26,250
there is one person Used with が and います to say that there is one person or that someone has one person of a particular relationship, such as one son or one older brother. 一人 is the counter for people.
JA: 公園に子どもが一人います。
EN: There is one tall man in the classroom.

子供が一人います

I have one child.

兄が一人が一人います

I have one older brother and one younger sister.

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.

トムにはが一人います

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.

息子が一人います

I have one son.