Can Rengar Pouncing be used as reaction defense and does he get assault?

Yes, Rengar, Pouncing can be played as a reaction during a combat showdown, but there are specific rules regarding his status as an "attacker" and his Assault keyword.

1. Can Rengar be used as "reaction defense"?

Yes, Rengar can be played at reaction speed during a combat showdown (a closed state).

According to the RiftJudge FAQ (ID 8779):

• Rengar possesses the [Reaction] keyword, allowing him to be played during a closed state (such as during a chain in a combat showdown).
• His card text explicitly overrides the standard restriction on where units can be played, allowing him to be placed at a battlefield you are attacking, even if you do not control it.

2. Does he get Assault?

No, Rengar does not immediately gain the benefit of his Assault keyword when he enters play.

According to RiftJudge FAQ (ID 5034) and FAQ (ID 6972):

• Timing: Rengar enters play at 3 Might without the "Attacker" designation.
• Cleanup: He only gains the "Attacker" designation and becomes 5 Might (3 base + 2 from Assault) during the combat cleanup triggered by his entering the board.
• Consequence: Because he is not Mighty (5 Might) at the moment he enters play, he does not trigger effects that care about a "Mighty unit" entering play (like Volibear's legend ability).

Summary of Sequence

1. You play Rengar, Pouncing as a reaction to a chain during a combat showdown.
2. Rengar enters the battlefield.
3. A cleanup occurs because a permanent entered the board during a showdown.
4. During this cleanup, Rengar gains the "Attacker" designation.
5. Once he has the "Attacker" designation, his Assault 2 keyword activates, increasing his Might to 5.

Note: This is my interpretation based on the card text of Rengar, Pouncing and the established FAQs regarding his interaction with combat cleanup and the Attacker designation.