"Red Echoes" is a Double-Meaning Title, if you think about it: Not only does it refer to the mythical "Red Echo Flowers" that set the plot in motion, it also refers to Chara, who — as a ghost — is a mere echo of themself.
Chara's amnesia and their hate for white chocolate sound eerily similar. Chara's memories make up a major part of them. If you replace the most important ingredient with something that tastes completely different, then it's not the same thing anymore. And we cannot claim to like something unless we accept it in its purest, bitterest, form. We can not claim to have saved Chara unless we accept their darkness too.
Flowey is able to read Gaster's reciting of periodic table elements and figure out what he spelled with their atomic symbols despite not seeming the science type. Why? Flowey spent years reliving the same week to see various outcomes. He even says in-game that some of the things he did involved reading every book and befriending everyone. Considering the Underground has a lab and Alphys is both a science and anime nerd, he has no doubt spent time reading and memorizing all the scientific information in the books the lab had.
Harlow Garamond is a monster who "returns all favours two-fold" and they live in a racist society. Given the Exact Words nature of their Check description, you have to wonder... is Aidrian their only victim? Who else have they "returned favours" to? Thankfully, Word of God says there are no villians in the story, so the worst-case scenario of Harlow being a Serial Killer is dodged.
Harlow is confronting Aidrian over causing a Literal Disarming. His check says he returns favours twofold. The resulting mental image is... notpretty.
Could be considered something more along the lines of Fridge Squick, but when considering Maverick, it's worth noting that he's obviously Toriel's son and Frisk would be his sibling since Frisk was adopted by Toriel. And since Aidrian is Frisk's blood sibling, that means that technically Aidrian and Maverick are brother and sister. So Aidrian is on a technical and unknowing level, pursuing a Brother–Sister Incest.
It's revealed that Maverick had a romantic relationship with his timeline's version of Chara, who was *also* adopted by Toriel. It's safe to say that he doesn't see this as a problem, as he didn't actually view Chara as his sibling, and thus wouldn't view Frisk- or Aidrian- as his siblings. Especially given the fact that he's from another timeline and doesn't exactly view the monsters from this timeline as his actual family.
In a "what-if" way, ClimaxStriker's plan to get Asriel's soul out of Chara by having them hug. The only reason Chara's not perma-dead from despair is because the rest of Asriel's soul is inside their soul, postponing their eventual death until they can be saved. Say, Climax's plan did work and Flowey became Asriel again through the hug. Chara would've had nothing keeping them alive. That plan would've indirectly had them killed and nobody except maybe Maverick would've known.
Aidrian's Action Girl and Hair-Trigger Temper become this with a little thought. We learn in a flashback that when Frisk told their dad that a girl in class punched them in the face, rather than comfort them and ask them if they were OK like a real parent would, he instead asked if they punched back and thenencouragedthem to do so if it happened again and to not let women walk all over them. Bad enough that Frisk was given that kind of advice, but who's to say that Adrian didn't get the same words, and unlike Frisk, actually took them to heart? Even worse, following that advice and hitting her Rage Breaking Point got her thrown out of the house, indirectly leading to her thinking that Frisk was dead.
Worse, think about what it did to Frisk. Chara was able to convince Frisk to hand them their body to protect them from monsters that would want them dead, and Chara only started killing out of self-denfense before Jumping Off the Slippery Slope. Again, a case where striking back when struck, or in this case, allowing others to do it for you went to hell in a handbasket. Frisk was too kind to do it themself, but they possibly wouldn't have taken until the Sans fight to stop Chara if they weren't raised to think that unnecessary force was the solution to conflict.
