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, rather handsome and 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.
Although rarely, Roswaal sometimes does make an appearance without his signature make-up. His face is noted to be very ghastly and pale and his eyes give off a gentle and rather welcoming expression, a polar opposite of what they give off when he's wearing make-up.
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.
The original Roswaal A Mathers was born some time before the Great Calamity. He was blessed with the Divine Protection of Sorcery which made him very skilled and apt with every type of magic however, back then, magic was very misunderstood and shunned on. Roswaal was treated as a mistake and an annoyance; ignored by everyone he met, his childhood wasn't full of happy moments. Some time in his late childhood, young Roswaal encountered Echidna who immediately recognized his insane magical power and talent and decided to take him under her mentorship. Throughout the years, Roswaal slowly began to unravel his magical talent and honed his abilities to near perfection although he was never able to achieve the same magical prowess as his mentor Echidna. By the time he was sixteen, Roswaal was already considered to be one of the top mages in the kingdom. Soon after, the Warlock of Melancholy successfully located Echidna in her Cloning Facility located in the Kremaldy Forest. Roswaal confronted Hector in order to buy Echidna some time but he ended up swiftly defeated and almost killed. In the last second, Echidna came to the rescue, fending off Hector and saving Roswaal's life. But the defeat Roswaal suffered was demoralizing and he was never able to fully heal himself again. Instead of achieving the pinnacle of Arcane Arts, he spiraled down the dark path of sinister magic, and over time, his mind had utterly changed. After Echidna died, Roswaal started to possess his descendants, with the purpose of bringing her 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, he has reached the zenith of magic being on par with his teacher Echidna if not even surpassing her. There is no magic that he cannot use or has difficulties acquiring, with the exception of Healing magic, which he was unable to learn. Despite being the original Roswaal, who was about as powerful as Emilia, he is mentioned to be much stronger than before, having been awarded the titles of "Red," "Green," and "Yellow" for having reached the pinnacle 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. He can reach the roughly 100km/h which is faster than a ground dragon and roughly on par with a winged dragon.
- 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.
- Four Colours of Mana: In the second arc, when Roswaal was preparing to fight Beatrice, he conjured four mana orbs of different colour. Tappei later stated the spell to be one of his original and extremely powerful attacks which he didn't bother naming.
- Various unnamed spells: Due to his insane magical talent and a wish to improve himself, Roswaal modified and created a lot of spells. Most of the spells he uses are at least heavily modified if not created by him from scratch. Despite that, he never bothers naming these new and/or modified spells.
Martial Arts: Roswaal is able to fight physically, using moves such as throwing techniques against opponents. He is still vastly weaker than some unarmed fighters like Garfiel, but he's non the less capable of holding his ground against very powerful opponents, as seen when Roswaal J Mathers defeated Sphinx using her martial arts since she was unable to use magic.
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 her 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 crueler than in the Web Novel, according to Tappei.
- Roswaal is said to be the strongest magic caster in Re:Zero. He also has one of the biggest mana supplies among any magic casters in the series.
- Roswaal L Mathers possesses a trump card that he used in his battle with Puck.
- Roswaal L Mathers was in fact created with the intention to make a child with the greatest suitability for magic. Apart from Healing Magic, it was a success as he is the most powerful mage in Re:Zero.
- He read all the books in the Forbidden Library.
- His goal of killing the Divine Dragon is not necessarily connected with his goal to resurrect Echidna as the author stated that even if he crossed paths with his mentor, he would not give up on his quest of slaying the almighty Dragon.