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 descendant's bodies.
Roswaal L Mathers is a tall, well-built man with long indigo hair with heterochromic eyes, the right one being blue and the left one being yellow. His eyelids are decorated by purple eye shadow, though the left eye's is longer. He wears a lavender coloured make-up which 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.
Roswaal cares about Ram and Rem, becoming concerned when Rem died of an unknown cause. Because of his relations with Demi-Humans, he has been rumored to have a thing for them, however 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 meant 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 lost. 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 Echdina, 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 as purpose to bring 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. Despite being the original Roswaal, he is mentioned to be much stronger than before.
- 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 amount 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 he used to make it snow in the Sanctuary.
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 for the first time shown 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.