Tropical Rain
Most mods install on top of a fresh copy of DDLC, free from ddlc.moe. Follow the author's instructions, and keep a clean copy of the game.
About this mod
Whoopee. You wanted to try and pursue the shy girl, Yuri, but you encountered a road block. Monika was now your girlfriend and you didn’t know what to do from there. You kept finding yourself thinking back to that purple-haired girl, but you know you shouldn’t have been. You tried to distance yourself but all that did was cause more heartache. You wanted to focus your attention on Monika, but you couldn't stop thinking about Yuri. ...Yuri was your girlfriend now. And you couldn't fathom what it was you’d just gotten yourself into.
Full credits
PumpkinbrainUS, Tune Eternal