Arataki Itto is a newer member of Genshin Impact's Geo family, an Oni with a big sword and a big personality who has quickly become a fan favorite. Since his release, he's proven himself to be a formidable close-range DPS, boasting impressive base stats and even stronger bonuses and buffs. Aside from the damage it can do, his Burst has left its mark as having one of the coolest casting animations so far--perfectly matching Itto's personality and style. However, the indisputable highlight of his kit is his Geo Construct: the incomparable and extremely adorable Ushi.
Itto is a pretty simple character to build and use, capable of being big damage without requiring any particularly demanding mechanics. His kit is well-suited to his character: straightforward, hard-hitting, and flashy. Despite his simplicity, Itto does specifically work best with Geo-based teams, meaning he does have a more constrained pool of optimal compositions than less-specialized DPS characters.
Geo Traveler
Despite the Traveler being commonly considered the worst of Genshin's sword characters, their Geo form is actually one of the best free-to-play options for low-cost players trying to build a mono Geo team for Itto. While Noelle and Yunjin are both Geo characters also offered for free (albeit temporarily, in Yunjin's case), neither are ideal options for Itto. Yunjin's passive talent gives a damage bonus based on the number of different Element types on a team, so it's best to avoid playing her with other Geo characters. While Noelle's healing and shield can be useful for Itto, she doesn't reliably generate any energy.
Itto thrives on teams with a solid source of energy generation, and the Geo Traveler can be a perfectly functional 'Geo Battery'. Using them for their particle generation also means their damage deficiencies are irrelevant, as their role on the team is just to feed Energy to the more active characters.
Bennett
Bennett is well-regarded as an incredible support, able to offer both healing and an almost unmatched Attack buff with his low-cost high-duration Burst. Since Itto can be a kind of 'reckless' DPS, in that he excels at melee-heavy close combat, having a convenient source of healing can be invaluable for keeping him upright during difficult fights.
Bennett is a good option for the flex spot in a triple Geo comp, providing non-Geo Elemental Damage and triggering Crystallize, but his relative similarity to Gorou keeps him from being ideal for this spot. Instead, Bennett works best with Itto in a 'Double Resonance' comp, wherein he and Itto are paired with another Geo and Pyro character. This would stack the buff from Geo Resonance with the Attack bonus from Pyro Resonance, a perfect combo for Itto.
Raiden Shogun
Counter to Bennett, Raiden works much better as the fourth option in a triple Geo comp than in Double Resonance. In a triple Geo comp, Raiden's ability to grant consistent off-field Electro damage makes her valuable for varying the damage type offered by this team, allowing for Itto to trigger different elemental reactions as well as Crystallize.
Raiden is a great character for Genshin's energy-hungry DPS like Eula or Hu Tao, and Itto largely sees the same benefits from her kit. She's the best Battery in the game, capable of unmatched Energy production, and can easily fuel Itto's powerful Burst--giving him a huge boost in potential damage output. With the addition of her buff to Burst damage, she can make Itto's Oni mode absolutely devastating.
Ningguang
Unless a player has a particularly well-built Traveler, Ningguang is generally the preferred low-cost 'Geo Battery' for Itto. Her Jade Shield provides a good amount of energy and its relatively short cooldown, which can be cut even further with her constellations, makes it a reliable source of particle generation. It can be used to block projectiles and eat some damage, but is infamously finicky to position. Instead, her shield is used best as a generator and for its Geo damage buff.
Ningguang, currently the sole Geo Catalyst in Genshin, can also be built as a great Sub or Burst DPS, filling in on-field while Itto is on cooldown. Her ability to output constant Geo damage is great against an enemy like the Golden Wolflord or for reliably triggering Crystallize. She also has some of the best range on a non-archer character, especially with her 'homing missile' Burst.
Fischl
When it comes to non-Geo options for Itto, Fischl provides all the benefits he wants in an easily accessible, 4-star package. In addition to being a great Battery, she's probably the most convenient source of off-field damage, as Oz is incredibly easy to upkeep consistently and requires basically no management. Her ideal builds are relatively straightforward, making it easy to turn Oz into a devastating Electro turret.
Fischl is a great flex option for a triple Geo team and provides a perfect option for a Double Resonance comp. Fischl and Beidou, one of the best duos in Genshin and the core of a variety of well-known comps, make an incredible pair for an Itto team. Beidou and Fischl are, independently, probably the best off-field DPS characters, and they are even better when combined. Providing not only a massive amount of off-field Electro damage but extra Energy from Electro resonance, they'd make formidable members of the Arataki Gang.
Albedo
Albedo is a great complement to Itto, filling in for his few deficiencies. His Skill's negligible cooldown means it can be cast constantly, providing both energy generation and consistent off-field Geo damage. When equipped with Cinnabar Spindle, which was given as a free weapon during his last rerun, his Skill can do an impressive amount of damage.
Where Albedo especially works to balance out Itto's kit is in his AOE damage output, which rounds out Itto's primarily close-range combat. His Burst is especially helpful for this, doing a wide range of Geo damage that can be especially helpful for clearing out mobs. With this, Albedo is great for keeping Itto from being overwhelmed.
Zhongli
Often considered the best polearm character in Genshin, Zhongli is probably an unsurprising recommendation. To his credit, Zhongli does work as a great universal flex option purely due to the quality-of-life benefits of his shield. As a close-range DPS with the claymore class's trademark slow attacks, Itto especially benefits from the security of Zhongli's shield.
As if his universal appeal wasn't enough, Zhongli is, appropriately, a particularly perfect match for almost every Geo character--especially ones who work best in mono Geo teams, like Itto. He's the undisputed king of Geo Resonance, allowing players to easily take advantage of the buffs Enduring Rock gives to shielded characters, and his Steles deal Geo Damage when placed near other Geo Constructs (like Itto's Ushi or Ningguang's Jade Screen). With his rerun coming up, Itto players might want to consider rolling for the inimitable Rex Lapis.
Gorou
Watatsumi's beloved general is basically tailor-made to be Itto's ideal support, and being released on the same banner means that most players will have both characters on hand. With Itto's attack bonuses scaling off DEF, Gorou's abilities are perfect for giving a significant DEF boost that can be maintained reliably for the duration of a fight. His Skill is the centerpiece of his kit, giving a variety of buffs to the active character, and the 10-second cooldown vs 10-second duration means that it is almost always available.
Although he can give Itto a DEF boost no matter what, Gorou's potential is maximized when on a triple or mono Geo team--which synergizes perfectly with Itto. With at least two other Geo characters on their team, Gorou's Skill will also provide an interruption resistance and a Geo Damage bonus. With the right combos, this can make Itto basically unstoppable.