C# (.NET Core), 58 57 bytes

s=>{int i=1;while(s[i]-s[0]<++i);return(char)(s[i-1]-1);} 

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Previous 58-byte solution:

s=>{for(int i=1;;i++)if(s[i]-s[0]>i)return(char)(s[i]-1);} 

The following algorithm must add using System.Linq; to the byte count and therefore is longer, but I quite liked it (52+18 bytes):

s=>{int i=0;return(char)(s.First(c=>c-s[0]>i++)-1);} 

And you also have a one-liner (45+18)-byte solution:

s=>(char)(s.Where((c,i)=>c-s[0]>i).First()-1)