Custom equipment

A select but growing number of characters have custom equipment that can be equipped by only them. These items can be found on the main equipment page, but this page covers them less as items and more as the story artifacts that they are.

Custom Weapons
Aka's Custom Knife (Aka, obviously): The first truly custom piece of equipment in the game. According to Aka, she uses an unusually light off-hand dagger, so in Stineford she requests that one be made custom. It requires proper ore, smithing, enchantment and money but is comparable to Durge equipment once finished. Only one can be obtained due to her fighting style.

Worn Whip (Yarra): Though at first apparently just a gift from the Empress, during Simon's route it is revealed that it has more of a history. The Empress used this whip when she was younger but no longer believes it is appropriate, as she is a ruler in power. She says it should be wielded by a revolutionary, so giving it to Yarra is a mark of esteem.

Sexy Sword (Nalili): Nalili's starting equipment. Though unique to her, it is not as good as the rest of the unique equipment and doesn't seem to have any story behind it. Given how succubi tend to use whips or sex toys, however, we can assume she had to have it created custom to match her fighting style.

Custom Armor
Lustplate (Simon): The ancient armor found in the optional dungeon that doesn't have a huge crotch hole. One of Simon's best pieces of equipment so far, and usable only by him presumably because it was made for an Incubus King.

Otherworldly Robes (Robin): After helping the cult on Aka's path, they offer to forge the demonic materials into a robe for Robin. The resulting piece of equipment is unique to her and an odd piece of equipment. Though it sacrifices many other stats, it has the highest magic defense of any armor in the game by a wide margin. Not necessarily a good general choice, but potentially very useful for magic-only fights.

Otherworldly Robes (Altina): If she is in the party, players can instead choose to forge the custom robe for Altina. It seems to be identical, just usable only by Altina instead of Robin.

Princess Armor (Nalili): Found in Nalili's room. Pretty decent armor for that stage in the game, though technically inferior to other options. But considering that equipment like the Chainmail Bikini is expensive, it might be better to leave the Princess Armor on Nalili so other succubi can use other equipment. This one is apparently made to flatter Nalili's figure.

Hilstara's helm (Hilstara): you get from Premium Steel if you have made an investment at the end of the Chapter 1/start of the Chapter 2. To the best of my knowledge, this is the very first customized item that depends from an Investment decision.

Misc
Simon's Old Equipment: Simon starts with five pieces of unique equipment: Simple Blade, Cracked Helm, Worn Armor, Weathered Shield, and Old Ribbon. It doesn't really seem fair to count them with the other unique equipment, because it's all equivalent to the lowest tier or worse. It sells for more than you would expect, but it remains to be seen if it has any other purpose. The Old Ribbon is the only item that boosts Luck thus far, making it useful for boosting the reliability of Simon's debuff abilities like Silencing Strike along with his critical hit rate, which makes Precise Strike even more dangerous.

Orc Equipment: All of Orcent's equipment can be used only by him. But considering that it's implied other orcs can use it, and that it isn't very good, it isn't really in the same category.

Orc Potion: This (apparently disgusting) potion is only effective if used by Orcent. It is notable because while Orcent can drink normal potions, these are far cheaper given the amount they heal. Basically only useful in the tournament on Yarra's route, or to keep Orcent alive during the optional boss on that same route.