Cheapest priced copy (Battlebond): $0.35 at Card Kingdom, near mint, their price list as of .
Impetuous Protege 
Creature — Human Warrior · 0/4 · Uncommon · Battlebond · #19
Partner with Proud Mentor (When this creature enters, target player may put Proud Mentor into their hand from their library, then shuffle.)
Whenever this creature attacks, it gets +X/+0 until end of turn, where X is the greatest power among tapped creatures your opponents control.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Battlebond | 19 | Uncommon | $0.35 | $0.49 |
| Battlebond Promos | 19s | Uncommon | — | $0.49 |
5 rulings from Wizards of the Coast
- 8 June 2018A nonlegendary creature can’t be your commander, even if it has a “partner with” ability.
- 8 June 2018The value of X is determined only as Impetuous Protege’s last ability resolves. Once that happens, the value of X won’t change later in the turn even if the greatest power among your opponents’ tapped creatures changes.
- 8 June 2018“Partner with [name]” represents two abilities. The first is a triggered ability: “When this permanent enters the battlefield, target player may search their library for a card named [name], reveal it, put it into their hand, then shuffle their library.”
- 8 June 2018Note that the target player searches their library (which may be affected by effects such as that of Stranglehold) and that the card they find is revealed, even though these words aren’t included in the ability’s reminder text.
- 8 June 2018If the greatest power among tapped creatures your opponents control is negative, X is considered to be 0. The same is true if your opponents control no tapped creatures.