Scrapy: return value if element belongs to a specific class

I am currently working on a Web Scraping project to scrape Data from a Newsletter Forum. For this i need to show if the comment was written by a staff member or by a reader/ customer. If the comment was written by a staff member i want to write "Admin", if not "Customer".

The comment was written bei a staff member if the span element contains username--staff. I already tried to use a if-loop in the items file but this did not work.

As you may have noticed i am pretty new to this stuff, so please forgive me if this was an incredible dumb question or if I did something wrong. I would be really thankful if someone could help me. :)

Here is my code:

Spider:

class ComputerBaseSpider(scrapy.Spider):
    name = "ComputerBase"
    start_urls = [
    'https://www.computerbase.de/forum/threads/in-eigener-sache-auch-wir-kommen-an-einem-consent-dialog-nicht-vorbei.1973328/',
     ]

    def parse(self, response):

        comments = response.xpath( '//*/div[@class = "message-cell message-cell--main"]')

        for comment in comments:
            loader = ItemLoader(item = CBCrawlerItem(), selector = comment)

            loader.add_xpath('Comment_no','.//ul[@class="message-attribution-opposite message-attribution-opposite--list "]/li[2]/a/text()')
            loader.add_xpath('Datetime','.//time[@class="u-dt"]/@datetime')
            loader.add_xpath('Comment', './/*/article[@class = "message-body js-selectToQuote"]//div[@class = "bbWrapper"]//text()[not(ancestor::*[@class="js-extraPhrases"])]')
            loader.add_xpath('Admin', '..//span[contains(@class, "username--staff")]')


            yield loader.load_item()

        next_page = response.xpath('//*/a[@class="pageNav-jump pageNav-jump--next"]/@href').extract_first()
        if next_page is not None:
            yield response.follow(next_page, callback=self.parse)

items file:

def clean_data(data):
    data = data.strip()
    return data

def remove_quotes(text):
    #strip the unicode quotes
    text = text.strip(u'\u201c'u'\u201d')
    return text

def user_check(user):
    if response.xpath( '//*/div[@class = "message-cell message-cell--main"]..//span[contains(@class, "username--staff")]') is true:
        return "Admin"
    else:
        return "Customer"


class CBCrawlerItem(scrapy.Item):
    # define the fields for your item here like:

    Comment_no = scrapy.Field(
        input_processor=MapCompose(clean_data),
        output_processor=TakeFirst()
        )

    Datetime = scrapy.Field(
        input_processor=MapCompose(),
        output_processor=TakeFirst()
        )

    Comment = scrapy.Field(
        input_processor=MapCompose(clean_data, remove_quotes),
            output_processor=Identity()
        )

    Admin = scrapy.Field(
        input_processor=MapCompose(user_check),
        output_processor=TakeFirst()
        )


    pass

This was my try:

loader.add_xpath('Admin', '..//span[contains(@class, "username--staff")]')

def user_check(user):
if response.xpath( '//*/div[@class = "message-cell message-cell--main"]..//span[contains(@class, "username--staff")]') is true:
    return "Admin"
else:
    return "Customer"