Down in the park with a fiend called Fi
or
Down in the park with a friend or five

The story:

In the song the first time this line is sung it
sounds singular and misleading. Later in the song
you can tell he says "With some friends of mine"
There are many other lyrics in this song that
can be misinterpreted because they are sung in a
electronic like voice.
I was reminded of this song because I read online
comic strips. One in King Features called "On the
Fastrack" has a character called Fi.