The Stamina Stat is a stat that adds to base health maximum of 100% in Strife.
You can potentially reach 200% health. The higher the stamina, the more likely the Punch Dagger will kill higher level monsters in fewer hits.
The additional health makes the Sigil more viable, especially as it consumes more health with each Sigil piece. The Surgery Kit's effectiveness becomes much more appreciated with the higher Stamina, since it will fully recover a player per use.
The maximum stamina value a player could have is 100.