Ainrand Leslie

"“On our side, he is also called Kidnap Leslie... Leslie the Kidnapper. In forests and wastelands, his home suddenly appears. You must never approach. Those invited in... never return.”"

- Wezel, volume 14+

Ainrand Leslie (アインランド・レスリー, Ainrando Resurī) is a mysterious entity said to have been an entrepreneur with unusual items. Many rumors revolve around Lesley and very little is known about him, some people even doubting his existence.

Something is known by all, however, he is the owner of the infamous and elusive Leslie Camp (レスリーキャンプ, Resurī kyanpu).

According to Wezel for those on the alliance of kings side, he is one of the Five Princes a group of undeads who were the No-Life King closest associates and is the only Prince whose whereabouts are unknown.

More importantly, Leslie is the one behind the volunteer soldiers amnesia, using a drug made from a relic to erase their memory, and likely also take everything too modern they could have on them.

Level. 13 Heart, Open, A New Door
Team Haruhiro and Kejiman stumble upon the Leslie Camp while on their escort mission, and from what Merry says at that time, it's likely that one of the people resurrected before her know Leslie.

Once in the tent of the camp, they hear Hiyomu's voice welcoming them inside and declaring herself as Leslie's disciple, but at that time the team doesn't recognize her voice.

From Kejiman ecstatic comments once inside the tent it appears like a lot of the rumors circulating on Leslie and his camp are true.

Moreover, the whole camp seems to be a relic and contains a number of other relics and a door to Parano adding

Level. 14 Parano-mania
He isn't named but as Hiyomu calls him master, the man appearing in the last chapter of Parano-mania likely is Leslie.

He "captures" team Haruhiro, team Io, Alice, and Reon, he chose to release most of them after erasing their memories in a situation similar to the beginning of the series but keeps 2 people for other use.

Abilities
As one of the Five Princes, he should have the ability to create undead, something only the princes and the No-Life King can do.

He is known to possess a wide array of relics, which we know for sure include: And if the Legends are to be trusted a  ring which can call tempest.
 * A memory-erasing drug-making relic which he uses to erase volunteer soldiers' memories.
 * A keyring of nine keys which seems to work as one-time use universal keys which once use turn into black mist.
 * The whole Leslie Camp also seems to be one big relic or at least under the effect of one as the space inside is impossibly vast and it's impossible to go back by the door used to enter it.

Quotes

 * "Alright then; I will show you how to use the ring. The price for it, however, will be your new wife."

Trivia

 * The name may be a reference to writer Ayn Rand.
 * There's a group calling themselves Lesliemaniacs which are looking for any clue to get to the camp.