When Hidetsugu and Kairi enters, draw three cards, then put two cards from your hand on top of your library in any order.
When Hidetsugu and Kairi dies, exile the top card of your library. Target opponent loses life equal to its mana value. If it's an instant or sorcery card, you may cast it without paying its mana cost.
Hidetsugu and Kairi is a value engine that smooths your draws on cast (drawing three, stacking two back) and pays off when it dies by burning an opponent for the top card's mana value and free-casting it if it's an instant or sorcery. The plan is to load your library top with bombs via the ETB, then sacrifice or blink Hidetsugu repeatedly to free-cast huge spells and drain opponents. You play a midrange-control game, controlling the board until your death triggers snowball into card and mana advantage.
Built-in card selection and topdeck stacking gives consistent access to your best spells
Death trigger generates a free big spell plus direct damage, rewarding repeated recursion
Excellent synergy with sacrifice outlets, blink, and reanimation
Evasive 5/5 flier that's a real clock on its own
Reliant on knowing/setting up the top of your library to maximize the death trigger
Free-casting an instant/sorcery is random unless you stack it, so it can whiff
Vulnerable to exile-based removal that denies the death trigger entirely
UB lacks easy ways to deal with resolved enchantments and artifacts
Cheaply returns a sacrificed Hidetsugu to chain ETB and death triggers.
Add fast mana (Dark Ritual, Mana Crypt) and efficient tutors (Demonic Tutor, Mystical Tutor) so you can set up the top of your library and reanimate Hidetsugu sooner. Tighten the sacrifice-plus-recursion package with Phyrexian Altar, Victimize, and Animate Dead to build loops. Lean into high-mana-value instants/sorceries that win when free-cast, and include scry/Brainstorm effects to guarantee the death trigger never whiffs.