Yandere, is the seemingly (sickeningly) sweet, kind, fun and loving guy (or gal, if you’re into that) who can turn deranged, aggressive and psycho when certain triggers are met, or sometimes seemingly at random. In summary the “nice guy” who can really turn out to be deranged, psychopathic stalker. Generally I do not like these kinds of characters and I will not let my mc’s romance them, provided we can catch onto their tricks quickly enough.
Tsundere on the other hand generally like to display a cold, uncaring or even downright hostile persona to the outside world but have a sweeter, kinder and more gentle side buried somewhere (deep) within them. If written well I like to romance these lovable jerks and some of the mc’s I play could even be classified as tsundere if looking at the relationship from the npc’s perspective.
In summary the iceman or (former) rival or both who you can defrost if you seriously try for a relation with them.