Moderate?
There were a few surprises here and there - in the last term there was actually a 5-4 conservative ruling where she voted with the conservative bloc and I believe Gorsuch was in the dissent, but overall she was probably the most left-wing jurist ever on the Court.
Did Souter ever vote with the right on anything?
On top of all that, he waited until a Democrat was president before retiring, five months into Obama's first term at the age of 69.
His wiki page still lists him as a "Republican."