together
偕 centers on the idea of being or doing something together with others, emphasizing shared presence or action.
偕 combines 亻 (person) with 皆 (all, everyone), suggesting people being together or acting in unison.
A person 亻 beside 皆 (all) — imagine one person joining a group so that everyone is together.
For カイ, picture a group of people together on a kayak trip: kayak -> カイ, and they paddle together.