What makes a good Grand Guardian in CK3?

Martial Skill Martial × 1.75 (max 50), Erudite +5, Diligent +5, Patient +5, Calm +5, Intellect Good 3 +10, Intellect Good 2 +10, Intellect Good 1 +5, Shrewd +10, Reclusive -10, Shy -10, Lazy -10 and Infirm -50 (the game's aptitude breakdown, patch 1.20.0.3).

What does the Grand Guardian give?

Terrible: Army Damage +1%, Army Toughness +1%; Poor: Army Damage +3%, Army Toughness +3%; Average: Army Damage +5%, Army Toughness +5%; Good: Army Damage +8%, Army Toughness +8%; Excellent: Army Damage +15%, Army Toughness +15%.

What tasks can the Grand Guardian do?

Guard the Capital, Shield the Realm and Instill Martial Discipline.

Celestial court

Grand Guardian

Grand Guardian is a court position in Crusader Kings III; its aptitude rests on Martial. At excellent aptitude it gives Army Damage +15%, Army Toughness +15%. Aptitude also comes from Erudite (+5), Diligent (+5) and Patient (+5). Base salary 0.5 a month; revoking it costs 200 prestige. Patch 1.20.0.3, from game/common/court_positions/types/00_celestial_court_positions.txt.

Quick answers (3)

What makes a good Grand Guardian in CK3?: Martial Skill Martial × 1.75 (max 50), Erudite +5, Diligent +5, Patient +5, Calm +5, Intellect Good 3 +10, Intellect Good 2 +10, Intellect Good 1 +5, Shrewd +10, Reclusive -10, Shy -10, Lazy -10 and Infirm -50 (the game's aptitude breakdown, patch 1.20.0.3).
What does the Grand Guardian give?: Terrible: Army Damage +1%, Army Toughness +1%; Poor: Army Damage +3%, Army Toughness +3%; Average: Army Damage +5%, Army Toughness +5%; Good: Army Damage +8%, Army Toughness +8%; Excellent: Army Damage +15%, Army Toughness +15%.
What tasks can the Grand Guardian do?: Guard the Capital, Shield the Realm and Instill Martial Discipline.

The Grand Guardian is the sword and shield of the realm, revered for their martial experience and strategic insight.

What it gives you, by aptitude

The holder's aptitude (see below) sets the level: levels start at 20, 40, 60, 80.

Terriblebelow 20
- Army Damage+1%
- Army Toughness+1%
Poor20+
- Army Damage+3%
- Army Toughness+3%
Average40+
- Army Damage+5%
- Army Toughness+5%
Good60+
- Army Damage+8%
- Army Toughness+8%
Excellent80+
- Army Damage+15%
- Army Toughness+15%

What makes a good Grand Guardian

The game's own aptitude breakdown for this position — every part it adds up:

Part	Aptitude
Martial Skill	Martial × 1.75 (max 50)
Erudite	+5
Diligent	+5
Patient	+5
Calm	+5
Intellect Good 3	+10
Intellect Good 2	+10
Intellect Good 1	+5
Shrewd	+10
Reclusive	-10
Shy	-10
Lazy	-10
Infirm	-50

The holder gets

Influence+3/month
Merit+10/month
Monthly Martial Lifestyle Experience+10%

Tasks

Guard the Capital
The Grand Guardian will take charge of the protection of the imperial capital's security.
Shield the Realm
The Grand Guardian will apply their formidable strategic skills to the defense of the realm as a whole.
Instill Martial Discipline
The Grand Guardian will endeavor to instill the accumulated lessons of their distinguished career on those who lack the privilege of experience.

Who may hire it (valid_position) (3 lines of game script)

is_independent_ruler = yes
government_has_flag = government_is_celestial
highest_held_title_tier >= tier_empire

As written in game/common/court_positions/types/00_celestial_court_positions.txt.

Who may hold it (valid_character) (5 lines of game script)

scope:employee = {
	grand_guardian_validity_trigger = {
		EMPLOYER = scope:liege
	}
}

As written in game/common/court_positions/types/00_celestial_court_positions.txt.

Shown when (is_shown) (3 lines of game script)

is_independent_ruler = yes
government_has_flag = government_is_celestial
highest_held_title_tier >= tier_empire

As written in game/common/court_positions/types/00_celestial_court_positions.txt.

Source: game/common/court_positions/types/00_celestial_court_positions.txt, common/court_positions/tasks, common/script_values and the English localisation, Crusader Kings III 1.20.0.3. Salary is the base before the realm-size and cultural-era multipliers.