Cecilia

Although Cecilia is not the powerhouse she once was, she is still among the select few green tome cavaliers in the game. Although she is not as strong as Gunnthra or Olwen, she is far more readily available and can be made passable without too much investment.
My Cecilia is built to counter things that she's already good at killing, thus Triangle Adept and Bowbreaker to augment her ability to take care of blue tomes and colorless/bows. Atk Smoke and Distant Defense are quite nice to improve her survivability as her bulk is low to begin with.
However, Atk Smoke is somewhat difficult to obtain, and so Savage Blow is a good alternative.
As with all cavalry units, she will function better with support from cavalry buffs, and is grateful for any other buffs she can come by.

(+10) (S-Supported)
The Commanding Officer of my army, the Mage General is a force to be reckoned with on cavalry teams. Near unrivalled power when fully buffed allows her to sweep aside a huge proportion of the cast. With Draconic Aura charged, that list grows even further, even defeating some of her counters. Drive Atk 2 provides additional support to all team compositions to give her use outside of cavalry teams. Hardy Bearing allows her to tear through pesky Vantage units fearlessly.

+SPD boon gives Cecilia an extra 4 speed totalling at 36 (+10) With horse buffs and swift sparrow, she reaches up to 46 speed which is enough to reliably double a large majority of the cast.

Cecilia is considered to be outclassed by Gunnthrá and Olwen as a Player Phase Blade Tome Unit which makes most players only use her as a Raven Tome unit, or as a budget Blade Tome unit. However Cecilia has superior attack and bulk to Olwen and Gunnthra and the only thing that holds her back is her pathetic speed. Therefore this build tries to give her a niche, but relies on +6 on each stat heavily. Olwen's and Gunnthra's Player Phase sets can work with Hone Cavalry only whereas this sets needs Fortify buffs (ideally from Reinhardt) too, because this Cecilia should one-shot her enemys.

To one-shot a larger amount of enemys Close Counter is the best option.

Vantage over Quick Riposte because Cecilia can one-shot most of her enemys and doesn't need the double attack. Moreover she isn't able to reliably stay in the Quick Riposte Range since 36/22 physical bulk isn't much but in most cases it's enough to survive one hit. Vantage utilizes her mediocre bulk and her monstrous attack (with Hone & Fortify Cavalry buffs she can reach 78 attack) to one shot most of her enemys. Once in Vantage range, she is close to be onstopable.

Hone Cavalry is the best Slot C option as you should always pair her up with someone that an give her great buffs. A "Tactic" skill is also an option but on a mixed emblem team she misses out on at least 6 Attack.

Glimmer is her best special due to the low cooldown of 3 (with Gronnblade penalty). Low cooldown is even more important on this set as Cecilia will always attack once because she's not doubling and therefore misses out on one or two special charges per Player Phase attack. She will deal nice damage even without Fortify buffs and therefore appreciate the 50% additional damage.

When it comes to IVs -Speed is a must for this set because this Cecilia set doesn't need speed at all to work. +Atk is imo the best option because more damage is always good. However one could consider +Def to get into Vantage range savely or +Res to use ploy seals.

Close Defense Sacred Seal saves her from one shots from powerful physical units. Alternatives are Atk +3 and Heavy Blade 3 or Res Ploy if you prefer +Res over +Atk or merged her quite a lot.

Owl tome along with Cavarly buff can make Cecilia already needed Cavalry team position much more viable to buff her overall stats, this can work fantastically well with as a team supporter.