Every player has a set of hidden attributes that impact their in-game performance. Most of these factor into the "star ratings" seen on player cards.
These attributes are normally not directly shown on the blaseball.com website. Purchasing an Apple shows a visualization for players on your team. Alternatively, there are several unofficial tools. A good one is abslve, which includes a "Forbidden Knowledge" section accessible by the link in the site footer.
Disclaimer: Almost all of this knowledge is based on correlations we observed. None of it is definitive.
Appears to turn fielded outs specifically into flyouts. Is countered by the opposing pitcher's suppression.
This is not used in the batting star calculation, but appears in the Vibes formula.
Ability to hit home runs. This is boosted by "Power" blessings (eg. Ooze, Mushroom).
Determines whether a batter ground out with a runner on base that has to advance, turns into a Fielder's Choice or if the runner(s) is/are able to advance on the out.
Represents "plate discipline" / drawing walks - determines how likely the player is to not swing at balls outside the strike zone.
Appears to determine if a batter's hit turns into a double instead of a single.
Also used to determine if a batted ball turns into a foul.
A "negative stat" (lower is better). Determines ball contact.
Higher patheticism correlates with higher strikeout rate, lower batting average, on-base percentage, and similar derived stats.
General "quality of contact" with the ball, higher thwackability reduces groundouts and flyouts and increases hits.
Tragicness is generated randomly, but often gets reset to exactly 0.1 at seemingly random points. Most current players have a tragicness of 0.1. The in-game effect of this is unknown.
How likely a pitch is to be inside the strike zone. Higher ruthlessness reduces walks and increases strikeouts. Along with unthwackability, is one of the primary components of pitcher performance.
Seems to reduce the power of batted balls - mainly lowers home runs, but seems to affect all types of hits.
Lowers hits allowed, increases ground outs and flyouts. The "counter" to batter thwackability.
Appears to convert fielder's choices into double plays.
Appears to counter batter buoyancy and converts flyouts to ground outs.
Unknown at this time, but there is weak signal that this is used in the stolen base rolls for a baserunner.
Determines likelihood of base stealing attempts, although not necessarily success rate. A player with high base thirst but low laserlikeness will get caught stealing often.
The most significant factor in the baserunning star formula, appears to be a general measure of "baserunning skill".
Governs runner advancement on hits, both how often and how many bases.
Appears to determine if a batter's hit turns into a triple instead of a single.
Related to runner advancement on an out, likely also related to sac flys.
Defense appears to be handled by the team as a whole, possibly taking the average of each of these values for the entire fielding team. On outs that name the fielding player, every player seems to field the ball at an equal rate, not correlating to that player's individual stats.
It's likely that the simulation first decides than an out happens, and then picks a player at random to attribute that out to.
Seems to "defend" against base theft.
Defends against extra base hits, holding runners to first base.
Controls fielding hits, turns batter hits into outs.
Related to steal attempts in some form.
Reduces baserunning attempts - unknown impact on success rate.
Determines whether or not a player is allergic to peanuts. An allergic player will experience allergic reactions during peanut weather, and a non-allergic player will experience yummy reactions instead.
Peanut allergies are not evenly distributed - As of the end of Season 11, approximately 60% of active players are allergic.
Cinnamon and pressurization
These two stats determine the lower and upper bounds, respectively, of the player's vibes, and have no known effect outside of this.
Pressurization also appears in the soulscream formula.
Ranges between 2 and 9 (inclusive), and determines the length of the player's soulscream (in multiples of 11 characters).
There is one exception to this: Chorby Soul has a Soul of 1777, which theoretically makes their soulscream 19,547 characters long. However, the formula breaks down after about 3,000 characters, after which it just repeats the word "undefined".
Ranges between 2 and 99 (inclusive). No known in-game effect, although this does show up on the public player page.
This seems to get rerolled occasionally, correlating with some (but not all) stat, modification, or Feedback changes.