Is this a flat or fold?

2/2 5 to call 8 handed

Hero rock straddles for 10 in utg, covers table
Utg+(passive player)1 limps 900 effective
Btn(unknown) opens to 45, 1000 effective
Sb(aggro reg) flats 200 effective
BB(huge fish) flats 500 effective
Hero in straddle w/K357ds???