I love this idea of "probabalistic advancement" for ttrpg mercenaries/sidekicks:
"At the end of a session where a mercenary unit earned XP... compare that XP to the XP needed to level. If the XP earned is greater than the total required, great, the unit levels. If not, convert it into a fraction of the XP required to level... and then into a die roll. If the roll is failed, the XP is lost."
No need to track XP b/w sessions, and the extra randomness is extra flavor.
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