Ferdibert

Ferdibert is the slash ship between Ferdinand von Aegir and Hubert von Vestra from the Fire Emblem fandom.

Canon
Hubert and Ferdinand are both originally students of the Black Eagle house in Garreg Mach. They are shown to have a mutual and outspoken dislike for one another; Hubert, for Ferdinand's "seditious" one-sided rivalry with Edelgard, motivated by his desire to be a worthy advisor to her; and Ferdinand, for Hubert's unwavering subservience to her in addition to his underhanded ways. In their C rank—the only pre-skip support they have together—they debate their polarizing ideologies for some time before dismissing each other as a lost cause and taking their leave. When forced to dine together in the mess hall during the school phase, they acerbically voice their disgust toward their seating arrangements.

In the Crimson Flower route, Ferdinand sets aside his antagonism with Edelgard in order to rise to his appointed role as her Prime Minister. Since the Empire made the executive decision to dissolve the nobility, he has reevaluated his standing in the world and reined in some of the zeal which made him unpopular during his scholastic days. Hubert also seems markedly less intense, perhaps from the success of the Imperial campaign. It is during this time that their B rank support takes place; in it, they are no less critical of each other with Ferdinand confronting Hubert over a threatening letter that he sent against Edelgard's orders, insisting that he should have petitioned her for a change of heart rather than slyly disobey her, and Hubert refuting the argument by citing Ferdinand's previous criticisms that he should "think for himself". Yet again, they dismiss each other, but not before indirectly suggesting a growing respect for their opposite by reproving themselves for expecting more from their conduct.

In their A rank, the two are shown to have taken great strides in improving their relationship. They take tea together—or rather, Ferdinand notes, tea and coffee, the bitter luxury better suiting Hubert's taste and marking yet another stark contrast between them. Hubert remarks that he never imagined that they would visit this way, then goes on to observe that, despite her many superior qualities, Ferdinand's optimism may exceed even Edelgard's as an asset. To this apparent compliment from the mouth of his greatest detractor, Ferdinand reacts with confusion, but Hubert continues to sing his praises, rhapsodizing his unyielding desire to improve himself, seek new knowledge, and push boundaries. Baffled, Ferdinand wonders if this sudden barrage of praise could be a deathbed confession. With a scoff, Hubert informs him that he prides himself on his ability to objectively analyze others and that his dramatic reaction is unnecessary. Ferdinand seems to understand this, which impresses Hubert. Still, distressed to hear such an outpouring of positivity from Hubert, like hearing "a snake sing an aria," Ferdinand insists with a blush that Hubert put it in writing next time, to which Hubert ("in the unlikely event") agrees.

Their A+ support begins with the two stumbling upon each other in the courtyards. Hubert greets Ferdinand, but a familiar scent brings him to an abrupt halt. Ferdinand confirms his suspicions—he has, on his person, a bag of fresh coffee beans. Hubert immediately questions his possession of them, recalling his distaste for them, which Ferdinand confirms. Hubert presses him again for an explanation, but Ferdinand is distracted by a smell which he himself recognizes: Tea. Hubert confirms as much, which earns him a jestful comment about the "fiendishness" of flaunting his favorite drink in front of him. Ferdinand guesses that it must be a gift for Edelgard, but Hubert declines to answer until Ferdinand explains the coffee, which prompts the Prime Minister to avert his eyes and blush, reluctant to answer. Hubert peers down his nose at him as he asks if it might be a gift for someone he fancies. Promptly, Ferdinand responds: Yes, a gift. For Hubert. Hubert chuckles, but then the reality of what was said hits him, and he repeats it for confirmation. Ferdinand assures him that he heard him correctly and insists he takes it, but with visible reluctance, Hubert declines. When questioned, Hubert explains that it's only because the thought of accepting a gift from Ferdinand "unbidden and unreciprocated" fills him with "revulsion". Therefore, he reasons, he can only accept it if Ferdinand accepts the tea as a gift in return. Ferdinand is pleased, but he states that it would be wrong to deprive its intended recipient of such a splendid token, and Hubert comments that it would be difficult to do so, considering that Ferdinand himself was the intended recipient in question. In a near mirror to Hubert's earlier reaction, Ferdinand begins to accept, but then reels back in surprise when he realizes what has been said and prompts for confirmation, which Hubert supplies him with, a rare and rosy blush on his cadaver-like cheeks. They share a laugh, then retire to partake of their gifts together.

In their paired ending, their image as a duo of political and dispositional opposites that "[bring] out the best in each other" is a hit among the public, earning them the sobriquet of the "Two Jewels". The blurb states that their shared popularity may have even made the Emperor jealous.

If the player chooses one of the two other houses and recruits Ferdinand (as Hubert cannot be poached from the Black Eagles), Ferdinand can potentially meet and kill Hubert on the battlefield. Engaging them in battle will prompt special dialogue in which they discuss the direness of the situation. Their tone during this short conversation suggests that there may still be some familiarity between them even on opposing sides.

Fanon
Fanon tends to depict the two during the latter half of canon, after the time-skip. Sometimes they will have had some sexual or romantic tension during the school phase, even despite their contentious relationship. There is usually great stress placed on the positive developments between them, and fics tend to be of a fluffier nature with the two being wholly in love with one another. There might be conflict in Hubert's primary loyalty to Edelgard or lingering feelings of love for her, concerns for Ferdinand's safety, or political complications. Hubert might see Ferdinand as a distraction from his goals, or he might find that Ferdinand fits comfortably alongside them.

It has strong competition from Edelbert (Edelgard/Hubert), which has strong basis in canon due to Hubert's loyalty to and canon feelings for Edelgard. Some Ferdiebert shippers resent Edelgard for this reason, believing Hubert's attachment to her is "unhealthy" and that Ferdinand is a better match for him. In turn, some more militant Edelbert shippers resent Ferdiebert for its popularity.

Trivia

 * The original Japanese text uses the phrase 肝胆相照らす to describe their relationship, which means to be profoundly compatible, inseparable, or "so close as to reveal to each other the bottom of one's heart."
 * Besides his S support with Byleth, his A+ support with Ferdinand is the only support in which Hubert can be seen blushing.