Overview
M500 Mod revolves entirely around her Mod 2 Skill, granting both M500 and other allies in her column 25 HP Shields every time a buff is applied to M500. While this can be applied through her own Skill 1, this is both far too infrequent and horribly late still having a way too long ICD of 10 seconds and very long cooldown.
Her Skill 2 causes any multiplicative DMG/RoF/EVA/ACC/Crit Rate/Armor buffs to grant M500 and whoever else is in her column to get 25 HP Shields. The ally in her column here is a requirement for this effect.
There are several Dolls with repeating buffs that can repeatedly trigger this with frequent enough intervals that M500 can help her entire column tank linksplash enemies, such as Doppels. This obviously requires the DPS for the echelon to be in M500's column and causes her to be a specialist role.