Samson is Filia's Parasite, whose being essentially replaces her hair and augments her overall strength. The two share a symbiotic relationship, and are shown to be on generally good terms with each other with Samson being both protective and supportive of Filia. Filia seems to have met Samson's acquaintance relatively recently, with her receiving amnesia upon becoming Samson's host. Reasons to how and why Samson got attached to Filia are kept vague, with the only known information being that it originates from an unknown event that occurred in her mysterious past. In her current state, Filia seems very reliant on Samson in their day-to-day life and seems to wholly trust him – likely a product of having no one else to turn to in hopes of regaining her memory and the knowledge of the ongoing threat of the Skullgirl. Likewise her loss of memories have left her with a very naïve personality, a trait that is implied to have been different before becoming a host to Samson.
Despite the twosomes close relationship and necessity to live off one another, their companionship can be strenuous at times, almost exclusively at Filia’s expense. Samson’s general brash and rowdy personality in addition to his bluffing and gambling bother Filia frequently, with her having no power over his actions and her protests generally succumbing to no avail. Similarly she is subject to the side-effects of his parasitism, where the consequences of elements such as eating, drinking and pain are symbiotic between the two of them (the former solely effecting Filia as the host of the relationship). It is also implied that Samson could be hiding important facts from Filia as hinted on several occasions, such as being hesitant on the subject of Painwheel knowing Filia after their fight during Story Mode. Likewise, Parasoul’s defeat quote against Filia "You're… being used." and Leviathan’s Story Mode quote "I won't idly watch you bring doom to another host!" suggests that Samson's intentions towards Filia might be less honorable then on the outset and gives the notion that he has manipulated his previous hosts prior to attain his goals.
Despite this, Samson has also shown genuine compassion for Filia and seems to have her best interests in mind – offering the option to stop the pursuit of finding the Skullheart if she so wishes during Squigly’s Story Mode (in spite of him seemingly pushing Filia towards it beforehand) and also attempting to defend Filia’s life at the end of Eliza’s Story Mode, even though Eliza’s intention is to replace Filia with a better vessel for him. In Filia’s unused alternate Story Mode ending, Samson also makes the implication that Filia had asked him to erase her memories in the first place. Samson furthermore provides shelter for Filia, persisting many of his contacts and “friends” to get him and Filia a place to sleep for the night, due to her not having a home of her own. In public he also typically remains hidden from sight due to knowing the prejudice parasites and their users receive (though it could be argued the latter two details would benefit Samson himself just as much as it would Filia).
Filia has repeatedly stated that she has wronged her friend Carol, AKA Painwheel, in the past and felt really guilty about it. In her story mode, she was suddenly attacked by her and the two fight. After that, Painwheel told Filia that she was Carol, to Filia's shock. At the Grand Cathedral, Filia came to Painwheel's rescue and defeated Valentine and Double, showing that she truly cares for Painwheel's well-being. Another instance of caring for her is that she wished on the Skullheart to give Painwheel a normal life at the cost of her own. It has been said that Carol was one of Filia's true friends pre-Samson since many people were afraid to be with her due to being a Medici family member. When the two face each other in battle, Painwheel shouts at Filia to get away from her; It is ambiguous whether Painwheel wanted to keep Filia safe or doesn't want to see her.
Squigly and Leviathan are considered to be rivals to Filia and Samson. While Filia and Squigly are friendly to one another, Samson and Leviathan are less than pleased to see each other. Leviathan usually criticizes Samson for his lack of tact and recklessness while Samson usually calls him an old worm. The girls and the parasites fight one another in Squigly's story mode when Squigly and Leviathan want to prevent Filia from wishing on the Skull Heart and become the Skullgirl like her mother did. Samson has made fun of Squigly's appearance and is distrustful of her since the undead can be controlled when there is a Skullgirl around. Despite the hostilities, Leviathan once considered Samson a friend and Squigly decided to let Filia's actions speak for herself instead assuming the worst of her for being a Medici. Filia and Squigly also teamed up on different occasions once they faced a threat (i.e. Double and Eliza).
Medici Mafia & Family (Including Lorenzo and Vitale)As Marie and some of the Filia's flashbacks indicate, Filia is a member of the Medici Family. More specifically, she is Vitale's niece and Lorenzo's grandaughter. She seems to know little of her past, but is quite saddened by the fact that her family killed the Contiellos once Double told her and Squigly.
Filia and especially Samson have an antagonistic relationship with Eliza. Samson hates her for killing Delilah, one of his cherished hosts, and many other misdeeds she has caused while Filia is determined to stop her evil ways. Eliza, on the other hand, has Samson as both her likes and dislikes in her bio. Although she believed he was not strong as he used to be and derided him for his devotion to Delilah, she still offered to give him a chance to be by her side and claim power so they and the other parasites could have a higher standing in society. She believed that Filia is unworthy to be his host and tries to find someone she deems stronger and worthier for him.
Because of her family lineage, most students fear Filia. Mrs. Victoria, on the other hand, tries to make sure that none of her students, including Filia, get out of line. It is said that Filia is the missing student she is searching for in story for the DLC campaign.