This low investment Ursula (with only Astra inheritance) reaches an insane 191 Atk with other low investment supports (Spur Atk inheritance) from allies and penalties on an enemy L!Eliwood.
191 x 1.5 effective damage = 286.5 (round down to 286)
286 x 1.4 triangle advantage = 400.4 (round down to 400)
400 x 2.5 astra special = 1000
This could go slightly higher especially when this Ursula only had a C support with M!Corrin, but I'm too lazy to grind for her supports just for that 2 Atk, or perhaps fodder 4 star Lilinas just for spur atk 3 (since my Kaden and L!Eliwood are using spur atk 2). I just stop here since 1000 damage is rather nice to look at.
Highest damage units by patch dates after SI (the character below replaces the one above):
Effie [highest atk unit, inherit an effective weapon] (Since SI update)
Exalted Chrom [+5Atk if below 99% HP, wins Effie by 2 Atk] ("The Branded King" banner)
Note: He stays at the top for the longest period of time (1 year 2 months) until Naga was released.
Gunnthra [Blizzard + Naga's dragon effectiveness] ("Naga: Dragon Divinity" banner)
Summer Lilina [highest Atk dagger unit, Broadleaf Fan, Naga's dragon eff] ("Summer Refreshes" banner)
S!Lilina, Halloween Dozla [same base Atk with superasset] ("A Monstrous Harvest" banner)
Note: H!Dozla can only do this with Cancel Affinity 3 on a Nagi with TA
Ursula, S!Lilina, H!Dozla [Ursula's "Cav eff + penalty" allows 1 more support than S!Lilina/H!Dozla to reach same damage] (Ursula's refinement update)
Ursula, S!Lilina, H!Dozla, Julian [Julian has same Atk as S!Lilina/H!Dozla] ("Heroes, Light and Shadow" banner)
Ursula, S!Lilina, H!Dozla, Julian [With Dominance from B!Micaiah] ("Bridal Beloveds" banner)
Ursula [+Atk from Trait Fruit, overtaking the other 3] (Sept 2020 update)
In any case, Ursula will remain at the top until a character with higher attack is released, along with a [penalty bonus + effective] weapon, or maybe a new type of weapon that scales attack from something even crazier than penalties.
I don't get Laevagames, and I don't mean it as a goal for achieving the highest damage, since I do it too, but because Laevatein herself is not the highest damage character at any point in time, as Gunnthra was released before her. I guess Laevagames does sound better than Gunnthragames.
Current 4.11.0 update: Under optimal conditions (but without fortress advantage), Ursula can do 1209 damage on an AR map, while S!Lilina/H!Dozla/Julian coming close at 1197 damage, with that said I can never achieve that, because I got no money. I even had to rely on Ursula's bonus week (+4 Atk) just to make this video.