Overview
Mona’s is a Hydro Support, although she applies relatively little Hydro compared to her competition. Her value is entirely through her buffing, which requires Hexerei: Secret Rite to be active to be truly great. Said buffing comes through the Omen effect she applies to enemies with her Elemental Burst, while her Hydro Application through her Elemental Skill. With her Hexerei passive, using a few of her Normal Attacks becomes important, but her own talent damage is generally unimpressive.
Elemental Skill
Mona’s Elemental Skill summons a Phantom on cast. The Phantom lasts for 5 seconds and will pulse around itself roughly every 1 second for a total of 4 pulses, with the final pulse being a slightly larger explosion. These will deal Hydro DMG and apply Hydro on the 1st hit, 3rd hit and last hit (assuming all 4 are able to hit the same enemy). If this final explosion hits an enemy, it will generate either 3 or 4 Hydro Particles (⅔ chance for 3, ⅓ chance for 4). Mona will not generate any particles if this explosion doesn't hit an enemy, even if the previous pulses did.
This Skill has a cooldown of 12 seconds, which is quite short but ultimately not impactful. The Phantom will also Taunt nearby enemies, helping you take a little less damage sometimes.
Elemental Burst
Mona’s Elemental Burst is on a 15 second cooldown and costs 60 Energy. On cast, itwill deal a bit of damage and apply Hydro, then apply the Illusory Bubble effect to all enemies hit by it. This will ‘trap’ smaller enemies inside it, though it won't otherwise affect bosses. Illusory Bubble has a duration of 8 seconds.
Additionally, Mona will apply the Omen debuff to all enemies at the same time, increasing the DMG Bonus of attacks that hit them. When an enemy with Illusory Bubble on them takes any kind of poise damage, the Illusory Bubble will ‘burst’ and give Omen a baseline duration of 5 seconds on the enemy that took poise damage. Some attacks do not do poise damage, and will not break the bubble, but most will.
Hexerei Buffs & Differences
When the Hexerei: Secret Rite effect is active, Mona will gain 2 effects, and both will require her to strike enemies with her Normal or Charged Attacks. The first will give her stacks of ‘Astral Glow’. She can hold up to 3 at once, and if a teammate caused a Vaporize reaction while Mona holds at least one stack, she will consume one to buff the damage of that attack by 5%. Normally attacks cannot consume more than 1 stack at a time, though there's a known bug that allows this to happen with certain attacks.
The second and more important effect states that when Mona strikes an Omen-affected enemy with a Normal or Charged Attack, she will extend the duration of the Omen by 2 seconds. The maximum extension is 8 seconds and requires 4 such hits, for a total of 13 seconds of Omen. This is a massive increase to her buff uptime, and is what has allowed her to make such a comeback.
With these Hexerei buffs active, Mona’s gameplay loop will generally involve using at least 3 of the 2s extensions after casting her Burst. Because Mona is a standard character, it should be relatively painless to acquire her C2. With her C2 Mona is able to extend the buff twice with a single Normal Attack, as the additional Charged Attack will also count for it.
Mona
Mona is a Standard 5* Hydro Support and a Hexerei unit. She received significant changes and upgrades with her Hexerei buffs as well as an additional buffing tool more recently with the 
Celestial Gift set. Focused around providing DMG% to allies who hit enemies affected by her Elemental Burst, she can also provide a relatively low amount of Hydro Application, which lets her take the role of a Vaporize Support under certain setups. As a Standard Banner unit, her strong C2 and C4 are also quite accessible and a significant boon to several of her teams.
Sandrone
Columbina