In the Neutral Route, there are multiple vessels to choice.
Depending the ending you seek, some vessels are better than others, and one is exceptional from the rest; but all depends in the choices of the player.
There will be 6 different vessels:
Frisk
Chara
UltSans
Velra
Tax
Lilium
Frisk
Soul of determination, and classic/original vessel in Undertale.
Is a balanced character, which is the most optimal one for everything, capable of switching of ending.
Is also the hard mode, what makes enemies being x2 stronger and all x2 complicated. But allows access to the True Pacifist ending.
Lv 1 stats:
HP 20
AT 10
DF 10
Every Lv obtains:
HP +1
AT +1
DF +1
Exp required:
From Lv 1 to Lv 19: +40 Exp added to Lv up.
Lv 1 to Lv 2: 40 Exp
Lv 2 to Lv 3: 80 Exp
Lv 3 to Lv 4: 120 Exp
From Lv 19, the Exp required increases exponentially.
From Lv 19 to Lv 20: 2000 Exp
From Lv 20 to Lv 21: 4000 Exp
From Lv 21 to Lv 22: 8000 Exp
Chara
Determination soul. The demon that comes when it's name is called.
Is specialized in DMG, and attacking with powerful strikes, improving quickly as an unstoppable storm.
The best choice for neutral genocides, capable of switching between genocides or neutral routes.
At Lv 20, gains the ability to use a variant of magic, similar and yet different, determined magic; which gives Chara the ability to deal Real DMG ignoring DF, throw slashes infused with determined magic, beams of determined magic, and determined magic knives.
Lv 1 stats:
HP 16
AT 14
DF 16
Every Lv obtains:
HP +9
AT +9
DF +9
From Lv 19 to Lv 20:
HP +999
AT +999
DF +999
Exp required:
From Lv 1 to Lv 19: +20 Exp added to Lv up.
Lv 1 to Lv 2: 20 Exp
Lv 2 to Lv 3: 40 Exp
Lv 3 to Lv 4: 60 Exp
Chara is capped at Lv 20, and to Lv up to Lv 20, is massive by the sudden gain is received in stats.
From Lv 19 to Lv 20: 1000 Exp
UltSans
The anomaly. Shouldn't exist. Experiment Number Eighteen.
Is still Asriel, but with the best of Sans, and the worst of both.
And is the only one with natural magic inside his soul. His role in a team would be tank and support.
Is the best choice for pacifist routes, and true pacifist. Is specialized in tanking, having god def.
Despite being the best choice for pacifist routes, including true pacifist, doesn't has access to the True Pacifist ending, which will only be a different true pacifist and not the canon True Pacifist.
Is the best option to use magic instead of physical attack. MAGIC>FIGHT
Very very hard to start a genocide, as incapable of hurting others or attacking directly.
In genocide routes, it will evolve into Hatred form.
In pacifist routes, it will unlock True Determination.
In Lv 20, losses Kr effect permanently, and gains Bl (acronym of Bleeding) and HC (acronym to Hate Corrosion).
HC has a side effect, that only fully applies if is the cause of death, as fully corrodes the body and soul, corroding the determination inside the soul and corrupting the body to use as infected as reanimated ally, or as item to be feed for the Hatred form. The body once infected can also be used at any moment as item to feed the Hatred form.
Corroded determination is instantly consumed, which implies a boost given while also unabling the refusal of the soul to persist after death. This makes the Hatred of UltSans stronger, and capable of killing easier.
Lv 1 stats:
HP 100
AT 1
DF 160
MG 20
Every Lv obtains:
HP +6
AT +1
DF +10
MG +20
After Lv 10, every Lv obtains:
HP +10
AT +1
DF +20
MG +40
After Lv 15, every Lv obtains:
HP +2
AT +1
DF +60
MG +80
From Lv 19 to Lv 20:
HP +40
AT +0
DF +80
MG +120
After Lv 20, every Lv obtains:
HP +6
AT +2
DF +80
MG +100
Exp required to Lv up:
Lv 1 to Lv 2: 60
Lv 2 to Lv 3: 120
Lv 3 to Lv 4: 240
Lv 4 to Lv 5: 360
Lv 5 to Lv 6: 480
Lv 6 to Lv 7: 600
Lv 7 to Lv 8: 720
Lv 8 to Lv 9: 840
Lv 9 to Lv 10: 960
Lv 10 to Lv 11: 1080
Lv 11 to Lv 12: 1200
Lv 12 to Lv 13: 1320
Lv 13 to Lv 14: 1440
Lv 14 to Lv 15: 1560
Lv 15 to Lv 16: 1680
Lv 16 to Lv 17: 1800
Lv 17 to Lv 18: 1920
Lv 18 to Lv 19: 2040
Lv 19 to Lv 20: 2300
From Lv 20, it increases exponentially. Multiplying x2 the Exp increased to the Exp required.
Lv 20 to Lv 21: 2400 (+100)
Lv 21 to Lv 22: 2600 (+200)
Lv 22 to Lv 23: 3000 (+400)
Lv 23 to Lv 24: 3800 (+800)
Velar
Perseverance soul.
The best option to play around between pacifist neutral routes, and very useful for tank role without any limitation.
Lv 1 stats:
HP 30
AT 6
DF 12
Every Lv obtains:
HP +6
AT +1
DF +2
Exp required:
From Lv 1 to Lv 19: +20 Exp added to Lv up.
Lv 1 to Lv 2: 20 Exp
Lv 2 to Lv 3: 40 Exp
Lv 3 to Lv 4: 60 Exp
From Lv 19, the Exp required increases exponentially.
From Lv 19 to Lv 20: 1000 Exp
From Lv 20 to Lv 21: 2000 Exp
From Lv 21 to Lv 22: 4000 Exp
Rax
Bravery soul.
The best DMG, the best option for genocide and true genocide.
Specialized in DMG.
Lv stats:
HP 28
AT 18
DF 4
Every Lv obtains:
HP +2
AT +4
DF +1
After Lv 10, every Lv obtains:
HP +1
AT +8
DF +1
Exp required:
From Lv 1 to Lv 19: +20 Exp added to Lv up.
Lv 1 to Lv 2: 20 Exp
Lv 2 to Lv 3: 40 Exp
Lv 3 to Lv 4: 60 Exp
From Lv 19, the Exp required increases exponentially.
From Lv 19 to Lv 20: 1000 Exp
From Lv 20 to Lv 21: 2000 Exp
From Lv 21 to Lv 22: 4000 Exp
Lilium
Kindness soul.
The best tank, specialized in pacifist route and protecting companions.
Lv 1 stats:
HP 40
AT 5
DF 20
Every Lv obtains:
HP +8
AT +1
DF +4
After Lv 10 obtains, every Lv obtains:
HP +10
AT +1
DF +8
Exp required:
From Lv 1 to Lv 19: +20 Exp added to Lv up.
Lv 1 to Lv 2: 20 Exp
Lv 2 to Lv 3: 40 Exp
Lv 3 to Lv 4: 60 Exp
From Lv 19, the Exp required increases exponentially.
From Lv 19 to Lv 20: 1000 Exp
From Lv 20 to Lv 21: 2000 Exp
From Lv 21 to Lv 22: 4000 Exp