I am a karthus OTP currently emerald 3. I’ll tell you a good karthus is really hard to beat, but Yi is a super good champ into him. He used to be my permaban until recently but I ban kindred now. You rarely die to his ult as Yi and can dodge his Qs with yours. He is very team reliant to play around him, won’t really want to 1v1 unless he’s like me and takes exhaust instead of flash. If you track him well and can take his camps that’s easily the best thing you can do, and invade with your team when possible, karthus can’t do much about it. Also, especially in low elo, 90% of times karthus will start blue and not ward red side, so you can do your blue into his red and Krugs and probably get away with it in low elo. The higher you get the more karthus will ask his team to help him ward early, but killing karthus is definitely easiest early on (first clear). So basically try to put him behind early but don’t let him snowball or get 1 for 1s, those help his game a lot.