#[05AB1E](https://github.com/Adriandmen/05AB1E), <s>7</s> <s>6</s> <s>4</s> 3 bytes

Code:

    l`-

Explanation:

    l     # Convert both elements to lowercase
     `    # Flatten the array
      -   # Loop over the second line of text and substract each char from the first character
            For example: "abcde""ba"- would result in "cde"

[Try it online!](http://05ab1e.tryitonline.net/#code=bGAt&input=WyJFIiwgIlRoaXMgaXMgYSBsaXBvZ3JhbS4iXQ)

Truthy is when the current letter is outputted. Falsy is when nothing is outputted.