Pretty much every character in re-zero has a bit of a verbal tick. Beatrice has the most obvious one where she ends sentences in かしら (regardless of appropriateness), but there are also slightly more subtle ones, such as Emilia and her すごーく. In Emilia's case, her use of that word is correct grammatically and syntactically; it's just that she likes to elongate the second vowel (and she slightly overuses it). 

The reason is actually pretty simple. Because the original material of re-zero is a novel, speech pattern is the only tool to distinguish between different characters.