Official Riftbound ruling

Does Kato give keywords he’s gained during play? What about Might bonuses from various sources like Equipment?

Yes. Kato gives his current keywords and additional Might equal to his current Might to the unit you choose. Later changes to Kato don’t matter; what the unit gets is locked in as the ability resolves.

Riot's question and answer, word for word, from theRiftbound Spiritforged FAQ

The card

  • Kato the Arm
    Kato the Arm

    Unit

    Deflect (Opponents must pay Any RuneAny Rune to choose me with a spell or ability.)

    When I move to a battlefield, give another friendly unit my keywords and +MightMight equal to my Might this turn.

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