Playing Gwent in The Witcher 3 is ridiculously addictive, but there's one drawback for people who haven't read the books or played the earlier games: a lot of famous characters, especially those featured on hero cards, aren't really explained at all. Arguably, it's just a bit more fun to trounce that one innkeeper at Gwent when familiar with the character card that cements the win.
So for those players who love using their Northern Realms deck but have no idea who Esterad Thyssen actually is, here's a handy guide to each and every one of the characters featured on Gwent hero cards (minus a couple of non-specific monsters and Geralt of Rivia himself). From Saesenthessis to Tibor Eggebracht, there are some big names from all across the Witcher franchise to cover.
Cerys an Craite
Fortunately, players have the pleasure of meeting Cerys (or, depending on choices during the game, Queen Cerys) in The Witcher 3's main campaign. Cerys an Craite is the "Sparrowhawk" of Skellige, daughter of a powerful Jarl and a great warrior in her own right. She's stubborn and confident, but also level-headed and clever; all qualities that make her an excellent leader.
Cerys's Gwent card is a strong 10 and can summon shield maiden cards to the board when played.
Cirilla Fiona Elen Riannon
Does Ciri need too much explanation? She's got such a rich history in the Witcher universe, and she's really the secret main character of The Witcher 3. As Geralt and Yennefer's adoptive daughter, Ciri has powerful magic and combat skills at her disposal, as reflected in her Gwent card: Ciri and Geralt's both boast a power rating of 15, the strongest cards in the game.
Eithne
Eithne the Silver-Eyed is the queen of the dryads and the ruler of Brokilon; although she's present in many of the Witcher novels, she has exactly zero appearances in any of the three Witcher RPGs from CD Projekt Red.
In the books, Eithne is known for two things: her remarkable wisdom, and her unbending hatred of humans. The only people dryads regularly allow to set foot in Brokilon are elves (hence Eithne's presence in the Scoia'tael deck), and they're a mostly reclusive people, drawing strength from the primal energy of the forest around them. Brokilon itself is just as magical as the dryads, and one tale says that dryads can see the future in the waters of the forest, hence how difficult is to surprise them.
In her Gwent card, Eithne's skills as a master archer make her part of the 'ranged' card category.
Ermion
Those who watched the recent Netflix Witcher show might be surprised to know that the character Mousesack has another name: Ermion. Similarly, those who have played The Witcher 3 may not know that these two characters are actually one and the same.
Ermion is a powerful druid, formerly an advisor to Queen Calanthe of Cintra before her demise and caretaker of Ciri; at the time of The Witcher 3, he's the advisor and friend of Skellige's king, Bran Tuirseach. Ermion and Geralt are also longtime friends since childhood, and when both later ended up at a duplicitous witcher tournament actually designed to eradicate the school of the wolf witchers, Ermion helped Geralt to escape with his life. He was also present during the fall of Cintra, though he managed to escape it.
The Witcher 3 hints at Ermion's long history with Geralt and Ciri, but it's never fully explained. Suffice to say, he's been around since the very beginning of the Witcher stories.
Esterad Thyssen
Esterad Thyssen was the king of Kovir and Poviss during the second war with Nilfgaard, and present in some way through many of the Witcher novels. Unfortunately, Esterad is killed before the events of the games can begin, though he's mentioned in The Witcher 2; when ambushed by assassins outside of his palace, Esterad chose to stay and fight for his wife's sake even though he could have saved himself by waiting for the guards. In the end, Queen Zuleyka survived without a scratch, but Esterad was mortally wounded.
Hjalmar an Craite
Hjalmar is the hot-headed brother of Cerys, son of Jarl Crach an Craite. He's another contender for the throne of Skellige in The Witcher 3 (and it's Geralt's decision who takes the crown) so players don't have to worry about not knowing his name when it pops up in Gwent. He falls under the ranged category, with a power level of 10 just like his sister.
Imlerith
Imlerith shouldn't be too hard for Witcher 3 players to figure out either, since that game is his only mention or appearance in the entirety of the available Witcher media out there. Sure, the Wild Hunt comes up in other places, but Imlerith himself is a CD Projekt Red addition who serves as one of the main bosses of The Witcher 3 as one of Eredin's generals. He's a close-quarters combatant with a score of 10 in Gwent, playing for the monster deck.
Iorveth
Iorveth has limited appearances in one of the Witcher books, and his only physical in-game appearance is in The Witcher 2. Thus, anyone who skipped to Wild Hunt may not know him: Iorveth is a factor in one of The Witcher 2's biggest choices, and siding with Iorveth radically changes the players' experience compared to a playthrough siding with Vernon Roche instead.
Iorveth himself is an Aen Seidhe elf and the commander of a Scoia'tael commando unit skilled in guerilla warfare; Iorveth's soldiers were fiercely loyal to him and he was a cunning commander, though outside of his unit, Iorveth was seen as a terrorist.
Isengrim Faoiltiarna
As it turns out, Geralt isn't the only "wolf" in The Witcher universe. Isengrim Faoiltiarna is only mentioned in the CD Projekt Red games, but he appears in a couple of Witcher novels as one of the most experienced members of the Scoia'tael, earning himself a moniker: the Iron Wolf.
When the second Nilfgaard ended, Isengrim and 31 other elven commanders who fought with Nilfgaard were surrendered to the Northern Realms for execution as part of the truce, but Isengrim himself managed to escape alone. By the time The Witcher 3 rolls around, Isengrim's whereabouts are unknown, though his reputation remains.
John Natalis
On the flip-side, John Natalis was a commander for Temeria, one of the Northern Realms. He shows up in one of the Witcher books and The Witcher 2, even temporarily serving as the regent of Temeria after King Foltest's assassination. According to Vernon Roche, sometime in between The Witcher 2 and 3, Natalis's forces were thoroughly defeated when Temeria fell, so his current whereabouts (or whether he's alive) are unknown.