Просто используйте ts.charAt(ts.length()-1)
:
jclass cls_String = env->GetObjectClass();
jmethodID mid_String_length = env->GetMethodID(cls_String, "length", "()I");
jmethodID mid_String_charAt = env->GetMethodID(cls_String, "charAt", "()C");
jint len = env->CallIntMethod(ts, mid_String_length);
jchar c = env->CallCharMethod(ts, mid_String_charAt, len-1);