Blue Magic — When Quistis Trepe enters, you may cast target instant or sorcery card from a graveyard, and mana of any type can be spent to cast that spell. If that spell would be put into a graveyard, exile it instead.
Quistis is a mono-blue spellslinger/value commander who turns any graveyard into your spellbook—when she enters, recast the best instant or sorcery sitting there with any mana. You durdle through the early game with cantrips and counters, then loop Quistis with blink and bounce effects to repeatedly steal high-impact spells (Time Walk effects, big draw, board-altering counters) until you snowball into a wincon.
ETB recursion that works off opponents' graveyards too, giving you flexible answers and free value
Mono-blue gives access to the deepest pool of counters, draw, and tutors
Color-flexible casting on the stolen spell lets you grab off-color cards milled or discarded by opponents
Cheap to recast (only 3 mana) so blink/bounce loops are very mana-efficient
Effect is dead if no good instant/sorcery is in any graveyard
Mono-blue means weak removal for permanents and no real ramp outside artifacts
Fragile 3-mana body that needs protection to chain ETBs
Can be slow and grindy without a tight combo finish
Recurs a key instant to the top of your library, fueling repeated graveyard plays.
Add reliable blink engines (Deadeye Navigator, Brago, Ghostly Flicker with Archaeomancer) and untap/mana enablers to convert ETB triggers into a combo finish. Improve consistency with tutors like Mystical Tutor and Merchant Scroll to ensure a high-value spell is always in a graveyard, and tighten the curve with cheap cantrips. Top end should include an alternate win like Thassa's Oracle so your value loops actually close the game.