ok let's see
Variable Heart Mai scales to be comparable to and probably stronger than beings that are relative to Take-Mikazuchi which is 4.86 Teratons of TNT (Small Country level+) which would put her probably closer to 6 Teratons.
True DxD Issei scales to be superior to Pseudo DxD Issei, which scales to be comparable to DxD Vali, who is 5,989 teratons (small country level +), which should put him close to 7 teratons
Variable Heart Mai has all the powers of her Remix Heart keys except what is granted by Azure, which should mean she loses Perception Manipulation, Regeneration (Low-Mid), Soul Manipulation, Healing, Regeneration Negation(High-Godly), likely Immortality Negation(Type 1, 2, 3, 4, 5, 6, 7, 8 and 9)
but he gets almost all of them back since in that key he gets Soul Manipulation, Healing and Regeneration Negation (High-Godly), Immortality Negation (1, 2, 3, 4, 5, 7, and 8), which means the only thing he really loses is Perception Manipulation, Regeneration (Low-Mid) and likely Immortality Negation(Type 9)
features: Forcefield Creation, Pain Manipulation, Illusion Creation, Telepathy(Able to know people's thought by tasting them), Information Analysis (By tasting, she can know what the object is and what is its purposes), Statistics Amplification (Able to increase her senses with her Grimoire), Power Nullification (Nullified Cipher's barrier), Shockwave Generation (Capable of blowing people away by bursting), Death Manipulation, Homing Attack and Instinctive Reaction, Resistant to Statistics Reduction, Soul Manipulation and Power Nullification
Issei has Telepathy, Cloth Manipulation, Statistics Amplification, Durability Negation, Resistance Negation, Attack Reflection, Absorption of Energy, Statistics Reduction, Power Nullification, Minor Spatial Manipulation, Paralysis Inducement, Resistance to Magic, Power Nullification, Death Manipulation and Soul Manipulation.
now let's see what each can do
Mai
Mai's Telepathy and Information Analysis works through her tongue so she would have to get close enough to Issei to lick him to use it and doing that against a guy whose main method of combat is melee doesn't sound like a good idea idea
Illusion Creation sounds good but I think between her Enhanced Senses, Extrasensory Perception and Telepathy of hers, I'm not sure how well they could fool Issei
Pain Manipulation sounds good, but Issei can fight while enduring the pain inflicted by the spears of light which is described as the maximum pain a demon can experience and as if the flesh is burning and the muscles and bones are melting, I endure the pain of absorb Vali's powers, pain that he himself claimed was greater than that of the spears of light and that even Ddraig, a soul without a physical body, could experience
her Statistics Amplification only amplifies her senses, not her power.
Issei resists Death Manipulation, Soul Manipulation and Power Nullification.
at this point there is only Shockwave Generation left, but unless it can generate internal vibrations I don't know how useful it would be
Issei
Telepathy with Pailingual(Breast Language), Allows you to hear the voice and communicate with the breasts of females. He can hear his thoughts and strategies and is useful for gaining insight into his plans, techniques and possible weaknesses, eliminating the chance of being caught off guard and allowing him to stay one step ahead of them. He ignores and bypasses the resistances and countermeasures to conventional telepathy, as it is simply mammary communication. The effects of it also extend beyond the biological sinuses
Cloth Manipulation and in my opinion a form of Deconstruction and Power Nullification with Dress Break(Clothes Collapse) in 3 flavors
Ver. B (Body): Issei can completely destroy the clothes of any woman he comes into contact with. As long as he has made contact with them, he can instantly activate it at any time by snapping his fingers. With the help of his Diabolos Dragon Deification, he can stop the targets' movements before activating the technique. Due to how he envisioned it when developing the technique, it works on anything that covers a woman, destroying Georg's device and some restraints attached to Asia, and the holy swords surrounding Jeanne's lower body.
Ver. A (Astral): Due to how he envisioned it when developing the technique, it works on anything that covers a woman, including magic and other techniques. Its effects also extend to non-physical techniques such as curses and mind control.
It should be noted that Issei is able to use the 2 previous forms at the same time
Dress Break DxD(Clothes Collapse: Dragon God Style): After undergoing Dragon Deification and hitting women with his energy, Issei can activate Dress Break from a distance without needing any prior physical contact.
Due to the conditions of this match, Statistics Amplification does not allow Issei to go beyond Low 6-B, but he can still amplify his speed to give him an advantage.
Durability Negation and Resistance Negation with Penetrate(Transmit): He can bypass objects that prevent him from hitting his opponents directly and any form of defense, whether physical or supernatural, including those that nullify powers on contact, allowing him to directly hit the target. core of his opponents, which can cause fatal damage. It can be used to see through solid obstructions when applied to the eyes, and can even be used to avoid attacks in certain situations; for example, a beam fight.
Attack Reflection, Absorption of Energy, Statistics Reduction, Power Nullification with Dividing Wyvern Fairy(Fairies of the White Dragon Emperor), Divide is pretty much useless since Mai has resistance to Statistics Reduction and Power Nullification, but Wyverns can still use Boost to a Issei amplify his stats beyond their limits and has Penetrate
Minor Spatial Manipulation, in DxD characters are able to warp and alter space with their raw power if they are strong enough, Triaina Issei could use Dragon Blaster which had this effect, in her CxC form she was shown to be able to do it with just her fists naked and later in the same way he can distort the space only with his pressure of his being.
Paralysis Inducement with Breast Power Wave Issei fires his red and black dragon energy at opponents. She has a special effect of completely stopping the movements of females.
Absorption of Energy with Nyuutron Beam Cannon (Super Breast Wave Cannon): Allows Issei to absorb energy from women's breasts by attaching the tail of her armor to them, with the absorbed energy used for a large-scale attack. Its potency depends on the size of the breasts involved. As a side effect, the female's strength will also be depleted, leaving her unable to initiate countermeasures.
one of her special attacks is Blazing Inferno of Scorching Flames: Issei releases flames that can never be extinguished and will burn continuously until the target is reduced to ashes or disintegrates.
With all that said, I'm leaning towards giving Issei the win.
