Also mocked with the trophies in the PlayStation 4 and Playstation Vita versions, see Effortless Achievement below.There's no special bonuses to this, but a yellow name run can be considered a reverse genocide run while in the latter you intentionally find every last monster in order to hunt them down and slaughter them, in the former you intentionally find every last monster in order to be nice to each one individually. To make every monster's name yellow, one has to encounter every monster with some exceptions, such as So Sorry at least once and fulfill all of their conditions. The game does provide a pacifist completionist goal, however in the true credits, monster names will be either yellow or white depending on if you've fulfilled a specific condition to make them especially happy or took an easy way out.Eventually, even Sans calls you out for doing this not because it's fun or challenging, but because you can taking the No Mercy route removes most NPCs, changes the fun music to slow, ominous chords, removes all the minigames, and makes all minor battles stunningly easy in short, it takes away everything about the game that makes people want to play it and makes it actively demoralizing, so Sans knows that the only reason anyone would have gotten this far is because they want the 100%. This makes him a pretty clear in-universe stand-in for the type of player who plays a game to death just to squeeze every ounce of gameplay and dialogue out of it, no matter how callous or cruel the actions they take are. He did everything he could differently in every way he could think of until he exhausted every possible outcome and began to see the people of the underground as predictable, scripted actors repeating the same dialogue over and over, so he mostly sat around waiting for something to happen. Flowey points out that he tried helping everyone, but eventually that got boring, so he tried killing everyone. This is outright discussed on the No Mercy run during the segment in New Home.And will "point out" the illogicalities of 100% Completion in the context of the world, to put it lightly. Certain NPCs will notice that you've played the game before.
If you want to see everything the game has to offer, you'll have to play through the game several times and take different actions, which is par for the course for most games with multiple endings however, both Undertale and its characters remember your previous runs