It might be intended to be Helen's father Arthur H. Keller as he was the one who sought out Bell to help his daughter in the first place. Bell was referred by J. Julian Chisolm who was recommended by Helen's mother
Helen also had three elder stepbrothers (they were paternal half-brothers, Arthur's children from a previous marriage) named William Simpson and James McDonald, so it could have also been one of them. Or possibly her younger brother Phillip Brooks if this was meant to take place much later (as Keller and Bell remained friends for life and visited regularly)
Given that Helen seems to be able to understand signs that Alexander does on her palm, this was likely not their first 1886 meeting, since it was the subsequent teaching in 1887 by Anne Sullivan (from a school Bell recommended, the Perkins Institute for the Blind which educated Laura Bridgman) which actually lead to her understanding signs more thoroughly.
Prior to Anne, Helen did understand some basic gestures from Martha Washington (the six-year-old daughter of the family cook) so it's possible Bell might've learned of these (by age seven, Helen had at least 60 home signs that the family was taught