Why is Wraith/Ghost Lewis in a suit despite wearing casual clothes when he was alive? Well, what are bodies usually buried in?
In the very quick confrontation of Freaking Out, Shiromori pulls out her scissors while Lewis shows off his fist of flames. In RockPaperScissors, a fist means a player is going for rock, and rock always beats scissors.
At the end of "Freaking Out," Lewis makes a Pimped-Out Car out of a passing truck, and you can see two peppers hanging from the rear-view mirror. They may seem out of place at first glance, but there is a reason Lewis has them. His beloved step-parents were named Mr. and Mrs. Pepper.
As shown in both "Ghost" and "Freaking Out", Lewis's heart locket has an enormous crack in it... because Lewis fell to his death.
There's also the fact that his death was caused (seemingly willingly) by his close friend, something most people would admit would break their hearts. It's just a little more literal in Lewis's case.
Not only that, the trailer Mysteryben made for one of the band's albums features Lewis fresh out of the incident that took his life; One of the most noticeable things is that one of the stalagmites is right where his heart was, and where the locket usually resides.
Lewis's character bio mentions he's a fan of Sailor Moon. In it, the Love Interest, Tuxedo Mask, is orphaned at a young age in an accident that also causes him to lose all his memories from before it. Recall that Lewis is a doorstep kid.
Not only that, even if he's the only member of the Pepper family without a spice or vegetable-evocative name, his transformation into a vengeful firey spirit makes him among the spiciest; a ghost pepper.
The warrior lookalike of Vivi in Hellbent sports a sword with a snowflake pattern, which makes sense when you consider Vivi's last name, Yukino, has the Japanese word for "snow" in it.
Aside from the obvious My God, What Have I Done? realization, why else would Lewis' heart locket break when he drops Arthur? Take note that colors are very important for the series, and among all the pink and black on Lewis there's one very prominent color that's not there anywhere else— gold. And considering who is the only person with gold and orange in this series, it can only mean that Lewis still cared about Arthur somewhere deep down.
When the locket breaks in "Ghost", he is pretty upset that he couldn't actually talk to Vivi, but he seems just as upset that nobody, not even Arthur recognized him. The color fading from his locket could be indicative of that as well.
Continuing on with the colors, Shiromori's Lewis-colored flower, the Dead-Beats, and Lewis' redecorated rig are all accented with gold. It gets way more promonent in Hellbent, where the truck's hearts ominously loom over the Mystery Skulls van and all the Dead-Beats' attention is on Arthur. Maybe the gold is a way of showing how focused Lewis is on getting revenge, to the point where he can see nothing but "gold".
This is going out on a limb, but perhaps Lewis never truly wanted to kill Arthur to begin with. Everything he does aside from him letting go of Arthur by the end of Hellbent could suggest an alternative motive; perhaps making him face his "crime" by putting him in that situation.
As for the act of dropping him, that could have something to do with Arthur's robotic arm containing that mysterious energy. It shuts down the Mystery Skulls Van, then disables Arthur's arm after he slams it into the dashboard, letting the van run again. What does Arthur do shortly before he's dropped? He grips Lewis' arm with his robotic arm. If it was an accident, that would explain Lewis' reaction and locket breaking again, this time turning black.
In Animated!Ghost, the gang runs almost literally into the blade of a possessed suit of armor. For a moment, it seems as though Mystery's head was cut off, before it simply pops back cartoonishly out of his collar. Considering his possible kitsune origins, it brings up the possibility his head did come off.
On that same note, given that Arthur was at the front of the group and that he was the one responsible for Lewis's death it's entirely possible that the blade was meant for him.
In Mystery's profile, the answer in regard to his favourite food, while humorous, is very clearly a diversion. If he really is a kumiho, or similar fox demon, then the actual answer is rather dark...
Given the quotation marks around the food too, it's possible he was speaking in euphemisms too: who has Mystery gotten a taste of during the animations we've seen? Arthur, whose behavior around the supernatural thus far has been noticeably more fearful than the other Mystery Skulls. Arthur is the group's chicken, and Mystery definitely knows what he tastes like...
Considering Lewis ran his old gang off the road in Hellbent by busting a tire on the Mystery Skull van, it's very possible he could of killed Vivi and Arthur purely by accident via a fatal car crash. Considering how his heart broke and went grey at Arthur or Vivi very possibly getting killed near the end, who knows how he would've reacted to the realization of killing his old friend and girlfriend by accident in a blind rage.