User blog comment:B214/Volume 22 spoilers/@comment-95.90.227.198-20170201191840/@comment-29963364-20170203092642

Here's some clarification:

Cases where Ophis gave people her "snakes"


 * Katerea Leviathan (increased her power to match Original Leviathan)


 * Shalba Beelzebub (increased his power to match Original Beelzebub)


 * Cruesery Asmodeus (increased his power to match Original Asmodeus)


 * Diodora Astoroth (increased his power)


 * Various Sacred Gear wielders (forced their Sacred Gears to evolve into Balance Breaker)

Cases where Ophis gave people her "blessing"


 * Asia Argento (increased her compatiblility with Dragons)


 * Irina Shidou (increased her luck)

And here is a special case that happened by just being around Ophis


 * Kunou (achieved a "Beast God" Form. Her fur turns white and her power increases by a lot).

As I'm sure everyone already knows, Dragons have a quality of drawing out the power in other beings. This happened when Rias and Akeno absorbed enough of Issei's "Dragon Energy" from his arm, which resulted in their [Extinguished Star] and [Raikouryuu]. And also when Kiba trained with Issei to the point his life was in danger, resulting in him achieving his Sub-Species BB [Glory Drag Trooper].

Ophis seems to take this quality to the absolute extreme. Her snakes seem to be equivalent to [Unlock Potential] from Dragon Ball Z. Her snakes unlock the hidden potential of the beings or objects she gives them to. Since Shalba, Katerea and Cruesery were decendants of the Original Satans, they had the potential to equal the Original Satans in power. So Ophis' snake "unlocked" that potential.

Her blessings seem to focus more an a "quality" of beings or objects, such as the case with Asia and Irina. Her snakes just "forcefully unlock" a beings/objects power, her blessings "maximize their potential."

Issei and Vali both had "hidden potential."


 * Issei (Ophis' power of "infinity," from when she remade his body)


 * Vali (He is a decendant of Lucifer, and the Grand-son of Rizevim, a Super Devil)

So when Ophis "blessed" them, their hidden potential was unlocked. Issei unlocked the power of "infinity" within him, and Vali unlocked his "Super Devil-Class" power of Lucifer.

When she blessed Issei and Vali, their Sacred Gears also reacted, resulting in the "combining" of the Heavenly Dragon's powers with their host's newly unlocked powers. That is the [Diabolos Dragon].