Fit, Join
ごう, がっ
あ, あい
None
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?
You need someone who will suit you. Someone who can join you on your life journey. So you call Grandpa Koichi (ごういち).
Note: For ごう reading mnemonics, we use ごういち (grandpa Koichi) because こういち is used for the こう reading. こう becomes ごう becomes "grandpa ごういち."