Backup Agent

When Backup Agent enters, put a +1/+1 counter on target creature.

Anointer of Valor

Flying
Whenever a creature attacks, you may pay {3}. When you do, put a +1/+1 counter on that creature.

Axgard Braggart

Boast — {1}{W}: Untap Axgard Braggart. Put a +1/+1 counter on it. (Activate only if this creature attacked this turn and only once each turn.)

Clockwork Beetle

Clockwork Beetle enters with two +1/+1 counters on it.
Whenever Clockwork Beetle attacks or blocks, remove a +1/+1 counter from it at end of combat.

Adaptive Shimmerer

Flash
Adaptive Shimmerer enters with three +1/+1 counters on it.

Codespell Cleric

Vigilance
When Codespell Cleric enters, if it was the second spell you cast this turn, put a +1/+1 counter on target creature.

Airlift Chaplain

Flying
When Airlift Chaplain enters, mill three cards. You may put a Plains card or a creature card with mana value 3 or less from among the cards milled this way into your hand. If you don't, put a +1/+1 counter on Airlift Chaplain. (To mill a card, put the top card of your library into your graveyard.)

Celebrity Fencer

Alliance — Whenever another creature you control enters, put a +1/+1 counter on Celebrity Fencer.

Ainok Bond-Kin

Outlast {1}{W} ({1}{W}, {T}: Put a +1/+1 counter on this creature. Outlast only as a sorcery.)
Each creature you control with a +1/+1 counter on it has first strike.

Clockwork Condor

Flying
Clockwork Condor enters with three +1/+1 counters on it.
Whenever Clockwork Condor attacks or blocks, remove a +1/+1 counter from it at end of combat.

Fylgja

Enchant creature
Fylgja enters with four healing counters on it.
Remove a healing counter from Fylgja: Prevent the next 1 damage that would be dealt to enchanted creature this turn.
{2}{W}: Put a healing counter on Fylgja.

Ardenvale Paladin

Adamant — If at least three white mana was spent to cast this spell, Ardenvale Paladin enters with a +1/+1 counter on it.

Basri's Acolyte

Lifelink (Damage dealt by this creature also causes you to gain that much life.)
When Basri's Acolyte enters, put a +1/+1 counter on each of up to two other target creatures you control.

Metastatic Evangel

Whenever another nontoken creature you control enters, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)