Items
The two items you get from the Legendary Artifact secret in Waterfall
Dog Residue and Dog Salad
The first character in the game you meet
Flowey
The only boss in the game with no Mercy button
Asgore
The FUN value 66 secret
(Bonus 100 points for the secondary secret)
Waterfall Hallway
(Gaster)
The only track to appear in both Undertale AND Deltarune
The secret item from the Tem Shop
Tem Armour
The character that causes Temmie to spare herself
Aaron
The two bosses who inflict Blue Soul
Papyrus and Sans
The name of Sans' beam attack
Gaster Blaster
The longest song in the soundtrack
Bird That Carries You Over A Disproportionate Gap
Acceptable answer: Bird
The item that becomes a full heal in "Serious Mode" fights (Sans, Undying, Asgore, Asriel)
Instant Noodles
The character that you can fight in both the Ruins and Waterfall
Moldsmall
The "boss" with -40000 DEF
Mettaton NEO
The characters who are aware of Save and Reset
Flowey, Sans, Asgore
The only song in the released soundtrack that doesn't feature in the game
Song That Might Play When You Fight Sans
The item that lowers Asgore's ATK and DEF
Pie
The character you can fight by staying in the Snowdin secret room for 2 minutes
Glyde
The name of Sans' debuff effect
(Bonus 100 points for the full name)
KARMA
(Karmic Retribution)
The two characters who featured in the Kickstarter video for Undertale
Sans and Papyrus
The main leitmotif used in the track "It's Raining Somewhere Else"
sans.
The item that allows players to break the game with cutscene skips and wrong warps
Punch Card
The two characters who dodge your attacks
Sans and Asriel
(Unlocking their spare and then killing them in a single hit)
Toriel, Papyrus, Muffet
The traits of the Seven Human Souls
(Bonus 200 points for the colours)
Determination, Bravery, Justice, Perseverance, Patience, Kindness, Integrity
(Red, Orange, Yellow, Blue, Purple, Green, Cyan)
The track that has a beta version used in the Kickstarter video
Bonetrousle