(This is a thread to give code feedback to the dev. If this is not you, you will probably be disinterested.)

Eevee has a problem - the way their stats are calculated leads to a much slower power curve than other characters.

They start with damage of 7.5 and tear delay of 20. This gives them a starting DPS on par with other characters, and is almost identical to a basic Isaac starting with Eve's Mascara. Therefore, they're not underpowered at the start of a run.

Practically any character, given two of "The Sad Onion", will halve their tears stat (regardless of where it starts). Therefore, two tears items should roughly double DPS.
Most characters start with base damage of 3.5 and a fixed damage multiplier, and most damage up items give about 1 damage. Therefore, three damage items should roughly double DPS.

Eevee has the same tears multiplier as most characters, but then there's a fixed +10 that's added afterward. For Eevee, two tears items brings fire delay from 20 to 15 - a gain of only one third. Additional tears items have no effect, as the game won't take tear delay below 5 in normal circumstances - and there's still the flat +10.
Eevee's high base damage is also due to flat addition. Their base damage of 7.5, plus three damage items, will generally come to around 10.5 - a gain of ~50%.

Taking both these issues together, a mid-game run with 2 tears up and 3 damage up items comes to about (1.5 * 1.33) = 2x power. Every vanilla character would see a gain of (2 * 2) = 4x power at this same point.
Therefore, a mid-game Eevee has only roughly half power, and gets worse as more items are added.

Fixing this is not difficult, however. Vanilla characters have multipliers to their FireDelay and Damage stats, rather than using flat changes; this lets them maintain the standard power curve.
Rather than using
you can write something like
which takes the existing stat, multiplies it by a constant, and rounds it. (add 0.5, then floor.)

Note that you could even load different constants depending on which Eeveelution is active. Hypothetical example: Eevee gets the slow-but-strong Shadow Ball (2x damage, 2x delay), Flareon gets the rapid but low damage Flamethrower (0.5x damage, 0.5x delay), etc. This would be a neat way of differentiating evolutions, giving sudden and dramatic changes to how the character plays.