Roswaal L Mathers (ロズワール・L・メイザース) is the lord of the Roswaal Mansion and a descendant of the original Roswaal. It is later revealed that he is in fact the original Roswaal, Roswaal A Mathers, who has been possessing his descendants' bodies.
Roswaal L Mathers is a tall, well-built man with long indigo hair and heterochromic eyes, the right one being blue and the left one being yellow. His eyelids are decorated with purple eye shadow, though the left eye´s is longer. He wears a lavender coloured make-up that accompanies his unusual outfit comparable to those similar to a clown or jester, consisting of a black hat, a top and bottom with different shades of purple, along with red and black boots and a cape. He also has a chain surrounding his right ankle.
He comes across as an eccentric due to his flamboyant appearance and odd speaking style, the latter which he shares with his grandmother. However, he harbors a more calculating and reserved side with a sharp political mind and gaze that reads others quite well.
Roswaal cares about Ram and Rem a great deal and holds them in high regard among his staff. However, he is unusual among the nobility for having such close relations with Demi-humans to the point that many are hired by him. This has led to unsavory rumors that have suggested he has a fetish for them. That said, Garfiel speculates this is actually because he's been gathering halfs to the Sanctuary.
In actuality, his body is possessed by his ancestor, Roswaal A Mathers, whose purpose is to meet his teacher Echidna again at any cost, even if that means sacrificing himself or those close to him, making Echidna describe him as being "a little too earnest." He has figured out that Subaru is looping through some method, causing him to challenge him, and subsequently lose. Though he has agreed to help him after losing, he has threatened to force him to loop if he makes a mistake.
400 years ago, Roswaal became Echidna's apprentice. However, Hector pursued Echidna, which caused her to protect herself in the Sanctuary for a time. Hector then encountered Roswaal and fought him. After Echidna saw Roswaal badly wounded, she decided to fight Hector. The outcome of the battle is unknown, but it is certain that Echidna survived. After that, Roswaal started to possess his descendants, with the purpose of bringing Echidna back.
Divine Protection (加護 Kago): Roswaal has the Divine Protection of Sorcery (魔導の加護 Madō no Kago), which makes him compatible with all six magic elements, enabling him to use the magic perfectly. Using this Divine Protection, Roswaal is considered to be the best in the kingdom in terms of mana usage, power, and speed, and is recognized to be as strong as one army.
Mage (魔導士 Madōshi): Roswaal is a mage capable of using magic of all six elements, though he claims to be unskilled when it comes to healing magic as he specialises in the use of offensive spells. Despite being the original Roswaal, he is mentioned to be much stronger than before, having been awarded the titles of "Red," "Green," and "Yellow" for having reached the pinacle of Fire, Wind, and Earth magic respectively. He can also use a unique form of rainbow colored mana.
- Flying Magic (飛行魔法): Roswaal is able to float or fly in the sky.
- Goa (ゴーア): Roswaal manifests a blaze of fire, which incinerates a foe.
- Ur Goa (ウル・ゴーア): Roswaal fires a large number of powerful fire bullets. A powerful flame scatters when the bullets land.
- Al Goa (アル・ゴーア): Roswaal lets loose a massive inferno. Al Goa is mentioned to be his most skilled magic.
- Altemillion (アルテミリオン): Altemillion is Roswaal's weather changing magic, which he uses to make it snow in the Sanctuary.
- Unnamed Fire Spell: An spell that has Roswaal conjure up 3 fireballs of differing hues, red, blue, and green. They can attack simultaneously in different directions and transform into towering flaming pillars or rolling waves of fire while expanding their size and hardness. The spell can break through other magics and is difficult to neutralize. While Roswaal is shown to normally start out with 3 fireballs, he can actually summon numerous amounts of them.
Martial Arts: Roswaal is able to fight physically, using moves such as throwing techniques against opponents.
Yellow Eye: Roswaal is able to use his yellow eye to break magic apart. This was first seen when Roswaal J Mathers fought against Sphinx and destroyed her Magic Circles with his Yellow Eye.
- Starting with his original body, Roswaal has been changing his middle initial every time he possesses a new descendant. His original initial was A, making his current body his 12th one.
- According to the author, Roswaal's birthday is September 16.
- All Roswaals' middle names follow the English alphabet thematically.
- Roswaal L Mathers has no sense of taste.
- Roswaal L Mathers has a habit of putting the crest of his family, a Hawk, into possessions and in clothes of people who live with him. It can be seen clearly in the sleeves of Emilia's outfit and in the cape of Subaru's knight outfit.
- According to Puck, Roswaal's power in on par with the Four Great Spirits.
- In the novelization of arc 4, Roswaal is more cruel than in the web novel, according to Tappei.