Jevil's Secret Space

- Jevil's name is likely a portmanteau of the words joker or jester and devil.

- Jevil is one of only five characters to currently have proper voice acting in Deltarune, the other four being Spamton, Tasque Manager, Queen, and Tenna. Jevil says "CHAOS, CHAOS," "I CAN DO ANYTHING," and "METAMORPHOSIS!" for normal attacks, and "BYE-BYE!" and "THE TRUE AND NEO CHAOS!" before his "final chaos." The voice acting, barring the "BYE-BYE!" voice line (which is identical in both localizations), is localized in the Japanese version as "Kaosu da yo, kaosu da ne!," "Nandemo dekiru!," "Henshin!," and "Shin no neo kaosu!," respectively.

- The line, "THESE CURTAINS ARE REALLY ON FIRE!" is almost certainly a reference to the term "curtain fire," an alternative name for the "bullet hell" sub-genre of shoot 'em ups, of which many attack patterns in both Undertale and Deltarune are reminiscent of and often compared to.

- Jevil is likely based off of the Joker from a standard 52-card deck. The card itself can add randomness and confusion to a game, and Jevil's character is likely a reference to it.

- Before Deltarune was announced, Toby Fox stated that Toriel's battle theme in Undertale, Heartache, was originally composed under the title of "Joker Battle" for an unused RPG idea. Since this RPG idea was later revived as Deltarune, it is safe to assume that Heartache was originally intended to be Jevil's battle theme, given Jevil's multiple allusions to the Joker card. In the final game, Jevil's theme, THE WORLD REVOLVING, is an original composition.

- The line, "HOW CAN YOU REFUSE, WHEN YOU ARE ALREADY PLAYING...?" may be a reference to the line "HOW DO YOU EXPECT TO OUTRUN ME. WHEN I AM ALREADY HERE?" spoken by the character Lord English from Homestuck, a webcomic series which Toby Fox has made music for.

- One of the Hypnosis actions "Susie spun something around," is a likely reference to various enemies such as the Urban Zombie or the Mad Duck in EarthBound, which have an attack with the text "(Enemy) made something spin around!"

- Jevil, along with Lancer, Rudinn, Hathy, Clover, King and the uncredited King of Diamonds, were designed by kanotynes.

- The carousel from the battle arena background is a picture of a Galloper Carousel from The Hoppings, a traveling fair from Newcastle upon Tyne.

- Jevil's voice clips are based on the "Joker" character from Fighters Destiny.

- The "3D" carousel in the background is actually a two-dimensional optical illusion.

- According to Toby Fox during Undertale's 6th Anniversary Livestream, Jevil's peculiar way of speaking was intended to emulate the poetically stilted syntax of localized RPGs.

- Toby additionally noted there was a bug at launch where Jevil's scythe attacks would instantly kill the player. This was initially not reported due to players presuming it was an intentionally difficult part of the fight.

- At the Fangamer FAQ with Spamton during the Spamton Sweepstakes event, Spamton implies he has known Jevil in the past.

- The flavor text, "The air crackles with freedom," also appears when fighting Spamton NEO.

- Jevil seems to be known to some of the people in the Cyber World for reasons unknown, though Jevil seems to know about Queen's existence beforehand and Queen herself seems to already know King as well.

- There is unused recruit info in the game's code for Jevil, suggesting that he may have been intended to appear as a recruit if he was defeated in Chapter 1, but the idea was scrapped during development.

- If you have either Jevilstail or Devilsknife equipped, using Susie's Bet30 ACT in Tenna's final boss fight will trigger an audio cue of Jevil's laugh. This seems to imply that, like Spamton, Jevil is also conscious and aware in your inventory.

- There is also an unused file in the game folder that contains a sprite of Jevil with is back towards the screen. This could be a nod to how there may have potentially been a scene with Jevil not facing us.

Congratulations, you are now a Jevil PRO!

And remember, you can do ANYTHING!