Catria is a member of the Whitewings, a Medonian squad of pegasus knights commanded by Princess Minerva. When Minerva's younger sister, Maria, is held in captivity to blackmail the elder princess into fighting against the Akaneian League, Catria and her sisters are forced to fight too. However, as Maria is rescued by Prince Marth, they become free to join his army and fight alongside Minerva again.
In her first appearance, Catria is a great unit that only comes with one real problem: a somewhat late recruitment accompanied with a low starting level. Unlike her appearance in the remake, Mystery of the Emblem Book 1, Catria shows up several Chapters later in this game(though still on the same map) due to some early game maps that were omitted in the remake. By the time you can recruit her in Chapter 14, you may have already filled out most of your army with units that are stronger than her at the time of her recruitment.
However, that is only a minor setback, as Catria comes with amazing growths that will quickly bring her up to speed with your other characters and eventually she will cap pretty much all stats except HP. Not only that, but she starts with 12 Speed and 10 Weapon Level, giving her access to most weapons in the game as soon as you recruit her and allowing her to Double Attack many enemies, making her low starting level and Strength of little consequence. Unlike later Fire Emblem games, Pegasus Knights and Wyvern Riders do not dismount indoors, making her one of the highest mobility units in the game.
If you put a little effort into gaining her a few levels, Catria will quickly catch up and even surpass your other units to wind up being one of the most dangerous units in your army.
Catria, and her sisters Palla and Est, are all Pegasus Knights recruited in Celica's route. When considering all of the Pegasus Knights in Gaiden, Catria and Est are both strong all rounders whose balance between Speed and Strength usually makes them the most offensively potent. Palla is strong and durable but slow for a Pegasus Knight, and Clair is very agile but fragile.
Catria is among of the best units available in Celica's route as well as in the entire game. She has very good starting stats for a level 5 unit, and her growths are excellent(with only HP, Luck, and Defense being average). Catria joins early in Celica's route at the same time as Palla, and starts 3 levels below her. She has mostly superior starting stats to her older sister, and by the time she catches up in levels she will likely have equal or better stats in everything except HP. Over time, Palla will fall farther and father behind Catria, maintaining marginally higher Strength and Defense and significantly lower everything else.
Catria is the best overall of the Pegasus Knight sisters. Est is technically slightly statistically superior if you level her up, but the only notable difference between the two statistically late game is that Est will wind up with much higher Defense. However, Est joins later in the game as an underpowered level 3 unit, and requires you to put in quite a lot of work to catch her up to speed. Catria is good from the moment you get her and remains good for the entire game. Due to Catria's early availability and comparable power level, she will likely be of better overall use to you than Est, though using both is a good option.
Catria is blessed with high growth rates in almost all areas, especially Skill and Speed; however, like most characters in the game, her Defense will usually end up quite low. Most of the time, she can do well in any class she is trained for. Catria can also perform the Triangle Attack with her sisters, regardless of her class, as long as ALL three sisters are not Snipers, because they need to be directly around the enemy to initiate the attack. However, each one can be an excellent Sniper.