Filo is Naofumi's second-closest companion after Raphtalia. The moment Filo hatched from her egg, she immediately became attached to Naofumi through imprinting. When Naofumi continued to raise Filo, her feelings for him grew even stronger to the point that Filo loves Naofumi very much that even when he made questionable decisions, she stood by him without even a second thought. It is through this that they both share their respective dislike towards Motoyasu. Her attachment also makes Filo worry that Naofumi might leave her. Filo's love for her master is more of a parental one instead of a romantic one.
Regardless, Filo becomes very invaluable to Naofumi that she is considered as important (if not right after) as Raphtalia. While Naofumi considers himself more of an owner to Filo, he treats Filo more like his own daughter. This treatment is similar to how he cared for Raphtalia when she still looked like a child. Filo is the second one to help Naofumi not be consumed by his cursed series, the Shield of Wrath.
Filo hates Motoyasu very much, due to him calling her "lame" and "a fat bird." Filo kicks him in the crotch every time they meet. After Motoyasu's female companions betray him, he has been trying to seek Filo's love, much to Filo's chagrin, causing her to run away from him.
Vassal Weapon Holders
When Filo first hatched from an egg Naofumi bought, Raphtalia was rather neutral towards Filo. However, when Naofumi started to give Filo special attention, Raphtalia started to become jealous of her. These feelings grow even further after Filo takes on her human appearance and whenever Naofumi compliments Filo for her appearance and actions. In return, Filo also sees Raphtalia as a rival for Naofumi's affections, to the point where she even states how she won't hand him over to Raphtalia, much to her chagrin. Whenever Raphtalia tries to share a moment with Naofumi, Filo will often (or wants to) interrupt in order to not be left out.
However, they also develop a healthy relationship where Filo sees Raphtalia as an older sister, though Raphtalia prefers to be seen more as a mother figure to better position herself with Naofumi who raised Filo. Raphtalia sometimes worries that Filo might pick up some of Naofumi's bad behaviors. In addition, their care for each other extends to working well together and performing combo skills in battle to protect Naofumi, who, in turn, protects them. They both have shared visible concerns for each other whenever one or the other is put in harm's way.
Fitoria sees Filo as her potential successor. While Filo is willing to get along with her fellow Filolial Queen, she will go against Fitoria, such as when Fitoria tests Filo by almost taking Naofumi's life and holding Melty hostage. It is through this test that Fitoria is willing to trust Naofumi and she blesses Filo to being the next Filolial Queen. This connection not only makes Filo stronger but also allows the two to communicate with each other. Fitoria also expects much from Filo, which includes watching over Naofumi.
While the two aren't often shown interacting, they became close as fellow party members. When they reunite after being separated in another world, Rishia shows her concern and care for Filo when the latter is being held captive for display.
She is Filo's best friend. Through their initial interactions, they became fast friends. Being in the same age group and Melty's interest in Filo and Filolials, in general, deepened their relationship. Filo was saddened by how Naofumi originally told her to no longer hang out with Melty.
When Melty ends up traveling with Naofumi again because of the Three Churches and Malty, Filo expresses joy in traveling with Melty again. It through this journey that they become even closer and care for each other's well being. This is first proven when Melty attacks Motoyasu for holding Filo hostage when Filo is unable to transform. Filo's trust in Melty becomes so deep that she is willing to listen and believe what Melty asks or says. This is first shown when Melty tricks Filo into playing hide and seek and the princess attempts a self-sacrifice to prevent a corrupted noble named Idol Rabier from finding Filo and her companions. In return, this caused Filo to want to rescue Melty. Their bond is strong enough that they're able to do a combo skill together.
Rat is the main learned professional within Naofumi's Village and is thus responsible as the doctor for the various inhabitants within the village. She first arrived after hearing about improved monsters such as Filo and often does various check-ups to make sure they are in full health.
Wyndia is the villager within Naofumi's Village responsible for training and maintaining the health of all monsters within the village. Part of this responsibility includes assisting Rat keeping monsters like Filo healthy. However, in addition to keeping up Filo's health, she is also partially responsible for raising Gaelion who is Filo's rival.
Both Filo and Gaelion are pet monsters who have Naofumi registered as their owner which means that they are often competing for affection. Being natural enemies as filolials an dragons, this competition is much more aggressive which usually ends up annoying Naofumi. However, they are both unified in their affection for Naofumi and desire to help whenever they can.
During their first encounter, the Demon Dragon Emperor had stolen Filo's levels and nearly killed her. During their second encounter, even though dragons aren't Filo's natural enemy in Glass' World, Filo still despises the demon dragon emperor after being revived. Whenever the demon dragon flirts with Naofumi, Filo takes it upon herself to protect Naofumi. However, the demon dragon doesn't consider Filo to be a particular threat or enemy.
Eventually, the Demon Dragon Emperor bestowed her blessing upon Filo which transformed her into one of the Four Heavenly Kings. Despite liking the ability to fly which came with this power-up, Filo hates using the demon dragon's power since it came from the demon dragon emperor.