Can you defy square up at repeat?

You can use Defy to counter Square Up.

Analysis of the Interaction

1. Defy's Targeting Requirements: Defy can only counter a spell that costs no more than 4 Energy and no more than 1 Power.
2. Square Up's Cost: Square Up has a printed cost of 4 Energy and no Power cost. Because it meets these requirements (4 Energy ≤ 4 Energy and 0 Power ≤ 1 Power), it is a valid target for Defy.
3. The Role of Repeat: The Repeat keyword functions as an additional cost paid when the spell is played. Paying the Repeat cost does not create a second spell, nor does it change the base cost of the original spell. When you target a "Repeated" spell with Defy, you are countering the single spell object on the chain, including both the initial and repeated effects.

Timing and Resolution

• You must decide to pay the Repeat cost before the spell is finalized onto the chain.
• Once the spell is on the chain, it can be targeted by Defy.
• If Defy is played successfully, the entire spell (including the repeated effect) is countered and moves to the trash per Rule 412.1.a. You cannot pay the Repeat cost after the spell has been countered.

Note: I could not fully verify this ruling against the retrieved rules.