hardcore
Extreme, uncompromising, or very intense with regard to a belief, hobby, or activity.
Er ist ein Hardcore-Gamer und spielt oft die ganze Nacht durch.
He is a hardcore gamer and often plays all night.

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Extreme, uncompromising, or very intense with regard to a belief, hobby, or activity.
Er ist ein Hardcore-Gamer und spielt oft die ganze Nacht durch.
He is a hardcore gamer and often plays all night.

As an English loanword, 'hardcore' is often not declined when it comes before a noun. Both forms, 'ein hardcore Fan' (a hardcore fan) and 'ein Hardcore-Fan' (a hardcore fan), are common in colloquial language.
Sie ist ein hardcore Fan dieser Band und reist zu jedem Konzert.
She is a hardcore fan of this band and travels to every concert.
The word 'hardcore' belongs to modern colloquial language and youth slang. It is rarely used in formal or official texts.
Die Fans der Mannschaft sind wirklich hardcore, sie unterstützen ihr Team bei jedem Wetter.
The team's fans are really hardcore; they support their team in any weather.
In the meaning 'extrem' (extreme), 'hardcore' is often used as part of a compound to describe a person or activity. Typical examples are 'Hardcore-Fan' (hardcore fan), 'Hardcore-Gamer' (hardcore gamer), or 'Hardcore-Training' (hardcore training).
Für dieses Hardcore-Training braucht man eine sehr gute Kondition.
For this hardcore training, you need to be in very good shape.
Be aware that 'hardcore' also has a very specific meaning in the context of pornography and describes the most explicit form. This meaning is widespread and should be understood to avoid misunderstandings.
Die Website wurde wegen ihrer Hardcore-Inhalte gesperrt.
The website was blocked because of its hardcore content.
extreme
'extrem' and 'hardcore' can both describe something as very intense or not moderate. 'extrem' is the more general word and fits many situations, including factual language. 'hardcore' is more colloquial and is often used for people, hobbies, opinions, or activities that are very committed, intense, or uncompromising. For example, people say 'ein Hardcore-Fan' (a hardcore fan) or 'Hardcore-Training' (hardcore training). For weather, ordinary difficulties, or neutral descriptions, 'extrem' usually sounds more natural.

Sie macht jedes Wochenende ein Hardcore-Training und läuft danach noch zehn Kilometer.
She does a hardcore workout every weekend and then runs another ten kilometers.
Im Kurs war die technische Aufgabe heute extrem kompliziert.
In the course, the technical task was extremely complicated today.