# k, 17 bytes
{`c$-1+x@&2=-':x}
[Try it online!](https://tio.run/##y9bNz/7/P82qOiFZRddQu8JBzchWV92qovZ/mrqGUmJSckqakrWSf2BQMJCqqAISILHUtPSMzKzsnNy8/ILCouKS0vKKyiolzf8A "K (oK) – Try It Online")
Explanation:
{ } /function(x)
-':x /create a list of differences/deltas
2= /check for equality with 2, result is boolean list
& /get indices of all 1s (there should only be one)
x@ /get letter at said index in our string
-1+ /go back one letter
`c$ /cast to character