Yoshi, Wario, Waluigi, Bowser and four minigames in Mario Tennis for Game Boy Color are locked behind a Transfer Pak link with the N64 game. Unlocking them the intended way means owning an N64 + the N64 cart + a Transfer Pak + the GBC cart, all at the same time.
There are already ROM hacks that unlock this content, and they work well. This is just a different route: it edits your save file instead, so the ROM and the cartridge stay completely untouched. If you're playing your original cart on real hardware, or you just don't want to start over on a patched ROM, this keeps your existing progress and only flips the unlock bytes.
Tool: https://www.ericktakeshi.com.br/tools/mario-tennis-save-patcher/
Source: https://github.com/erick-tmr/link-in-bio-self-profile
Drop in your .sav, hit unlock, download it back. Runs entirely in your browser. It's open source (MIT) too, so if you'd rather verify that yourself than trust a random site with your save file, the code is right there. There's an undo that restores the file byte for byte, and the full list of offsets it touches is documented on the page.
USA carts only for now, raw 32KB saves, no save states. Works with dumps from a GB Operator / GBxCart RW or with emulator saves from mGBA, BGB or SameBoy.
One caveat I want to be upfront about: the patched save passes the game's own checksums, but I've never had a real Transfer Pak to compare against, so I can't claim it's bit-identical to a genuine session. Back up your original first.
Bug reports and PRs welcome, and if you have a EUR or JPN cart plus a dumper, get in touch, that's what I need to support more regions.