Suit
Fit, Join
The radical suit and the kanji suit are exactly the same! This kanji can also mean join which makes sense. When something suits you well, like a hat or something, it joins together with you quite well too. Join me, my hat! You suit my head well. It just fits, you know?
ごう, がっ
あ, あい
None
You need someone who will suit you. Someone who can join you on your life journey. So you call Grandpa ごういち.
Note: For the ごう reading mnemonics, we use Grandpa ごういち because こういち is used for the こう reading. So just remember that ごう comes from Koichi's grandpa ごういち.