Rogue
Base Class: Rogue
Azul has been taught to sneak, hide, watch, inform, but she also knows more senior members of the Argent Watch become knights that do not hide their light - at least not all the time.  She envies knights, but knows she can't emulate them - this would be her own path to serving Sol Invictus and Astrea and protecting the innocent from the dark.  
After rereading the descriptions of Sol Invictus and Astrea I am guessing the Argent Watch is viewed with fear and or suspicion by some of the more straight-laced clergy who don't think sneaking around is consistent with the teachings.  Just shine a bright light on everything and all will be well!

Level 3: Bursts of Light

You harbour a wellspring of radiant energy within yourself, represented by your Bursts of Light, which fuels powers you have from this subclass. 

You have four Bursts of Light, which are expended when used. You regain one expended Burst of Light when you finish a Short Rest, and regain all of them when you finish a Long Rest.  In addition, if you roll Initiative with no Bursts remaining, you regain one Burst.  You gain an additional Burst of Light at Rogue levels 7 and 15.  The Lightbringer table shows the number of these Bursts of Light you have when you reach certain Rogue levels.

Lightbringer Bursts of Light
Rogue Level   Number
3   4
7   5
15   6
     
     
     

You regain one of your expended Bursts of Light when you finish a Short Rest, and you regain all of them when you finish a Long Rest.   In addition, if you roll Initiative with no Bursts remaining, you regain one Burst. 

Radiant Strike. When you hit a creature and do damage with a Sneak Attack, your weapon emits a bright flash of light.  You do one additional weapon damage roll and the target must make a Wisdom saving throw or be frightened of you until the end of its next turn.  Undead, Fiends and Aberrations have disadvantage on this saving throw.  The DC of the saving throw is 8+PB+CHR.

Mote of Healing. as a Bonus Action, or as a Reaction when you, or an ally within 30' of you, take damage, you send forth a mote of healing light that restores half your Rogue level (rounded down) +CHR hit points.

Radiant Sight. You can open your awareness to detect Undead, Fiends and Aberrations, no action required. For the next minute or until you have the Incapacitated condition, you know the location of any creature of those types within 60 feet of yourself, and you know its creature type. Within the same radius, you also detect the presence of any place or object that has been consecrated or desecrated, as with the Hallow spell.  You can also see normally in Dim Light and Darkness—both magical and nonmagical—within 60 feet of yourself for the same duration.

 

Level 3: Bearer of Light

You know the Light Cantrip.  Charisma is your spell casting attribute.

Level 3: Resist the Dark

You add your CHR modifier to saving throws you make against Charm or Fear causing spells or effects.

Level 3: Chastize the Malevolent

When you make a Sneak Attack, you can change the damage type to Radiant.  At Rogue level 7, you can change the damage type of all your weapon attacks to Radiant.

Level 9: Abjure the Malevolent

When you hit an Undead, Fiend or Aberration with your Sneak Attack, you can use a Burst of Light to attempt to [Tooltip Not Found] it instead of damage it.

Sancity of the Light

You are immune to Fear and Charm effects.  Attacks against you by Undead, Fiends and Aberrations are made with Disadvantage.  

Level 17: Relentless Light

If you miss with an Attack Roll against an Undead, Fiend or Aberration, you can immediately make another Attack against the same target, with Advantage.  You may do this Proficiency Bonus times between Long Rests.

Relentless Light

If you miss with an Attack Roll against an Undead, Fiend or Aberration, you can immediately make another Attack against the same target, with Advantage.  You may do this Proficiency Bonus times between Long Rests.