Alfonse is one of the 3 main characters of Fire Emblem Heroes that the player is introduced to at the start. He's the prince of Askr, and a reliable, cool-headed young man. Throughout the story of Heroes, the player learns more about Alfonse and the world he inhabits since they're a mysterious strategist from another world.
The game's twists and turns engulf Alfonse and company in mystery with every new chapter, which leads to shocking revelations about Alfonse himself. He's one of the more recognizable characters from Heroes itself and has made a place for himself among the protagonists of Fire Emblem since Heroes has made Nintendo the most money by far.
Askr Trio
He's the strongest unit of the core 3 the player receives because of his Folkvangr skill. This skill buffs his attack and defense when he's at low health. He can inherit similar more powerful skills from units like the Black Knight to make him even stronger. It isn't saying much to be stronger than his sister and Anna, but at least players will get some utility out of him before they draw better characters from the gacha. For a 3-star unit, he's decent compared to others.
Cameo Appearance
Alfonse makes a guest appearance in Dragalia Lost, another Nintendo-licensed gacha game. He's playable in the game from the Lost Hero event, and is a sword character that uses the light element. It's interesting to see a character from another gacha game appear in one as a guest because Alfonse isn't nearly as recognizable as other characters from the Fire Emblem franchise. Other characters, like Marth, were included in the crossover event, but the others were all exclusive to the mobile game.
Lord Alfonse
Alfonse is actually the first lord character to originate outside of a Nintendo console release of Fire Emblem ever. Lords are generally classes associated with the main characters of a Fire Emblem game, so it makes sense Alfonse would have it.
Many lord class characters are royalty of some sort like Corrin from Fire Emblem: Fates or Edelgard from Three Houses. Alfonse is the prince of Askr so the lord class fits well. It's strange that he's classified as a lord considering classes don't have much bearing on Fire Emblem Heroes but it's a nice bit of trivia.
Askran Duo
Askran Duo is a unit variation of Alfonse that features him and his sister together in one unit. His duo skill buffs all of his stats and protects him from follow-up attacks on the enemy's turn which is extremely valuable for certain challenge levels. He's less niche than his other variations so this unit is definitely worth investing time into.
The biggest thing he lacks is the Quick Riposte skill which can guarantee follow-up attacks on an enemy phase. Having this means he can take out enemies from any range, even when it's not his turn.
His Quest To Become King
As prince of Askr, Alfonse stands to inherit the throne and take over the government of Askr. It isn't until Book III that Alfonse can meet with his father after the hectic events of earlier chapters. When he meets with the king, it becomes clear that Alfonse isn't sure that he even has what it takes to be king.
From the beginning of the story, Alfonse has been putting himself at risk and leading the Order of Heroes with the player. The king scolds him for this saying that if the king were to die, Askr would have nobody to lead and he needs to consider this moving forward.
Trust Issues
Alfonse plays a darker sort of hero's role at the start of Fire Emblem: Heroes in comparison to others. He has trust issues from events that took place before the events of the game involving Zacharias, the main antagonist of the first chapters of the game.
Zacharias' real name is Bruno, and he was childhood friends with Alfonse. The two trained together and even taught Sharena how to fight. Zacharias reveals he was a spy for a rival empire and tries to destroy the Order Of Heroes. This betrayal hurt Alfonse and he now has trouble trusting anyone, including the heroes recruited to fight the Emblian empire.
Voice Actors
The English voice actor for Alfonse is Ray Chase who does other voices for Fire Emblem. Chase is also the voice of Roy and Gaius in Heroes. Chase is a well-established voice actor who's done work for other video game, anime, and cartoon franchises.
Some of those include Ryomen Sukuna from Jujutsu Kaisen, Bruno Bucciarti from Jojo's Bizarre Adventure, and Rhys from Borderlands 3. It's always fun to hear a voice and recognize it from somewhere, but Chase's voice kinda seems like it's everywhere.
Fire Emblem Cipher
There's a Fire Emblem card game called Cipher that features Alfonse in a lot of different cards. Some of the artwork for his cards comes directly from the mobile game, but a lot of it is original artwork for Cipher.
Generally, Alfonse appears in his classic costume wielding his signature weapon, but some cards have special costumes and weapons for certain sets and events. Alfonse is the most represented Heroes character in Cipher as well.
Smarter Than He Seems
Throughout the stories in Fire Emblem Heroes, there are many characters who hide their identities, pose as others, and have mysterious backgrounds. Almost every time one of these people pops up, Alfonse seems to have a bit more insight about it than the other characters.
For example, when Lif appears, he seems to be a version of Alfonse from another world, but when this is proposed, Alfonse seems skeptical. It's later revealed that Lif is an ancestor from another timeline who arrived in the future which would affirm Alfonse's suspicions. An even better example is when Loki takes Anna's appearance. Before it's revealed who she is, Alfonse says that the player character probably figured out who she was well before, hinting that he did too.
Epithet Trivia
Alfonse's origins as a prince and title of Lord are not his only noteworthy titles in Fire Emblem: Heroes. On top of these titles, he also has the title of Spring Prince in his "Hares At The Fair" event variation. Alphonse shares this epithet with Xander from his "Spring Festival" variation. There aren't many shared epithets in the game, and the Japanese version of the game actually gives them different titles, so fans wonder why that isn't the case for the English release.