Kaho, Minamo Historian

The enters-the-battlefield trigger looks like a tutor, but it is really a constraint disguised as one: the three instants you choose are locked into exile, castable only through the activated ability, one per tap, and only if you can pay an X that matches a spell's mana value exactly. That activation is the entire engine. Because it reads "a spell with mana value X," each tap commits you to one number and casts one exiled instant whose cost matches it; you cannot chain three off a single payment, and once an exiled card is cast, it is gone (it goes to the stack and resolves into the graveyard like any other instant). The real cost is tempo, not arithmetic. Kaho asks you to spend a turn naming X and tapping a fragile 2/2 body just to release one of three free spells from the vault, which means the instants worth exiling are the ones whose impact justifies a full turn's setup: a counterspell held up, a removal piece, a game-ending burst. Raw card-advantage filler does not pay her rent. The body invites attacks and the payoff arrives slowly, which marks this as a design from an era when a four-mana Wizard could reasonably ask for several turns of investment before doing anything. What endures is the toolbox texture: you stockpile three answers the turn she lands, then deploy them one at a time, paying mana to convert the right one for each moment until the vault runs dry.