# 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