Hapi

Weapon: Demonic Tome

Assist: Positional Assist / flexible

Special: Moonbow / Ruptured Sky

IVs: +Atk / -Def or -HP

Passive A: Atk/Res Catch / flexible

Passive B: A/R Far Trace / Lull Atk/Res 

Passive C: Joint Drive Atk / flexible

Sacred Seal: Atk/Res Solo / Blade Session / flexible

• Demonic Tome allows Hapi to deal high damage with a pre-charged Special. It reduces the cooldown of any Special she has equipped by 1, and charges it by another 1 if the cooldown is at max at the start of her turn, allowing her to immediately charge a 2-cooldown Special like Ruptured Sky or Moonbow every turn and making those great choices. 
• The ideal boon is +Atk; Hapi’s ability to deal damage is more important than any of her other stats. -Def or -HP are good choices of bane; -Def is arguably better as Hapi’s Def is already terrible and she will die in one hit to many physical attacks regardless of the bane. 
• The most important thing to consider for Hapi’s A skill is Atk boost; her native Atk/Res Catch is a great option as it provides +7 Atk and Res if her foe’s HP is 100%, or if they have any penalty, including status effects like Panic, and +9 if both conditions apply. It may also be worthwhile for Hapi to boost her Spd instead of her Res in order to better pierce Spd-based damage reduction such as from Spurn; Atk/Spd Catch is arguably best in slot with a potential +9 to both stats. 
• Hapi has multiple good choices of B skill. A/R Far Trace debuffs her foe’s Atk and Res by 3 while also allowing her to move after combat if she did not move a full 3 tiles before combat. Lull Atk/Res is a similar option that forgoes the Canto effect in favor of neutralizing her foe’s bonuses to Atk and Res during combat, increasing her effective damage and decreasing damage taken. Lull Spd/Res could also be used to improve Hapi’s effective Spd and avoid reducing her foe’s Atk, which will slightly reduce the damage of Ruptured Sky under some circumstances, but generally not by more than 1 point. However, this is generally not significant and the Res debuff is what’s most important in any case. Windsweep is another option that can deny counterattacks from foes Hapi has greater Spd than, but this is generally not reliable unless her Spd is increased through an A skill or seal, or both. 
• The C skill is flexible, with Joint Drive Atk being a reliable option that increases Hapi’s Atk by 4 if an ally is within 2 spaces, as well as increasing the Atk by 4 of any allies within 2 spaces of her. This is one useful option, but many skills can work. 
• The seal is best used to further increase Hapi’s Atk and potentially Res. Atk/Res Solo is a great option if she will generally not be adjacent to an ally while in combat. Blade Session provides a larger maximum boost to Atk (as well as Spd), up to +9 if two allies are on the map and have ended their turns before Hapi. 

Weapon: Demonic Tome

Assist: No assist / flexible

Special: Moonbow / Ruptured Sky / Iceberg

IVs: +Atk / -Def or -HP

Passive A: Atk/Res Catch / AR-D Atk/Spd / flexible

Passive B: Lull Atk/Res / Windsweep

Passive C: Fatal Smoke / flexible

Sacred Seal: Blade Session / Quickened Pulse / flexible

• As a ranged cavalry with two kinds of effective damage (beast and dragon) and a tome that allows her to pre-charge a Special, Hapi is a solid choice for an Aether Raids defense team that can deal high damage to many common annoying offense units like Fallen Edelgard. She can immediately charge a 2-cooldown Special like Moonbow or Ruptured Sky with the effects of Demonic Tome, or opt for Quickened Pulse seal in order to charge Iceberg, which adds extra damage equal to 50% of Hapi’s fantastic Res, generally more than other Specials. 
• The ideal boon is generally +Atk to improve Hapi’s damage. -Def is the best bane in most circumstances, since Hapi’s Def is almost unsalvageable, while -HP is also fine. 
• Hapi’s native Atk/Res Catch is a great choice of A skill that provides a hefty amount of Atk and Res under fairly easy conditions, which is especially useful if she’s using Iceberg. AR-D Atk/Res provides a slightly larger boost so long as at least 4 non-Fortress Defense Structures are intact. Alternatively, she can opt for an A slot that boosts Atk and Spd in order to better use Windsweep to deny counterattacks from slow units, and potentially reduce the amount of damage reduction from foe skills like Spurn, though depending on the context and the unit in question they may still be 10 Spd faster than Hapi and receive the full damage reduction. 
• Lull Atk/Res is a great B slot that reduces Hapi’s foe’s Atk and Res by 3 while also nullifying any buffs the foe has in those stats, which could be an extra 6 or 7 points. Windsweep can be used in combination with Spd-boosting A and S skills in order to deny counterattacks from slow physical units like Brave Hector or Fallen Edelgard. 
• The C skill is flexible, and there are many potentially useful options. Fatal Smoke makes Hapi’s foe unable to heal during combat, and if she survives, applies a penalty after combat that prevents them from healing until the end of their next action. This is a highly useful effect against units that rely on healing, which many Aether Raids carries do. Options like Joint Drive Atk, Panic Smoke, Distant Guard, or others may be great depending on the team context. 
• Blade Session is an amazing seal that buffs the user’s Atk and Spd by 3, 6, or 9 depending on whether 0, 1, or 2 allies are alive and have ended their turn. On an Aether Raids defense team with a slot-boosting mythic such as Nott, the unit in the 7th slot ends their turn immediately if all units on the team are active, guaranteeing a boost of at least 6 under those conditions. Quickened Pulse is also a great option in combination with Iceberg, as mentioned previously. Depending on the needs of the team, another Atk-boosting seal like Atk/Def Form or Life and Death could be used.