Adult Link walks into Raccoon City and fights the way he does in Ocarina of Time. He replaces Leon and Claire on screen. His model, textures and animations are read from your own Ocarina of Time ROM when Melty sets the mashup up, so nothing from either game ships with it.
What you get
Link from your ROM: the N64 model with the Master Sword in his left hand and the Hylian Shield in his right, moving with OoT's own idle, walk, run, targeting and attack animations.
Z-targeting: press Z to lock onto the nearest zombie (OoT's Switch targeting) and Link turns to face it.
Sword combat:V swings the Master Sword. Keep pressing to chain OoT's three-hit combo. While targeting, a still slash is a vertical cut, and walking forward turns it into a thrust. X while targeting is a jump attack.
Spin attack: hold V to charge, release to spin.
Hylian Shield: hold C. Hits from in front do no damage.
OoT's damage numbers: a Raccoon City zombie takes hits like a Redead (8 HP). The Master Sword does 2 and a jump attack does 4, so a zombie goes down in four slashes.
Knockback: every hit shoves the zombie away from Link, with a short hit-stop when the blade connects. Finishers, jump attacks and spin attacks shove harder.
Ocarina of Time's sounds, from your ROM: Master Sword swings, the spin attack, sword strikes, the Hylian Shield block, and Link's own attack shouts and hurt cries.
RE2's own guns and controls still work.
You need
Resident Evil 2 (2019) on Steam.
Your own The Legend of Zelda: Ocarina of Time ROM, US version 1.2 (Rev 2) (.z64, .n64 or .v64). Melty asks for it the first time and reads Link from it.
Good to know (v0.1)
Link is drawn over the game picture with flat-colored triangles, the way the N64 model's textures average out, so walls don't hide him and he isn't lit by the scene.
Zombies get knocked back but don't play a flinch animation.
Keyboard controls only for Link's moves.
Link hides on the pause, inventory and map screens.
Credits
Ocarina of Time's file layout, animation tables and damage values come from the zeldaret/oot decompilation project.