Kafir is always harbi i.e. the enemy of Islam, like the dare-devil (Al-Shaitan) is called Kafir (Wa Kana min al Kafireen). Do you think Shaitan is a mere disbeliever or he is an open declared enemy?
Again you are saying that kafir is a non-believer. It is true but he is a different kind of non-believer. he is a combatant, hostile disbeliever. If you were a Muslim then I could have given you a very good clear example of a Kafir.
shaitan/satan is called a kafir because because satan rebelled against God, thatās one definition, there are others
kufr : āthat which covers the truthā, infidelity, blasphemy, rebellion against God, disbelieving in the Qurāan or any of the tenets of islam ( rejects the faith ).
someone who commits kufr is a kafir
hereās some verses :
But they
who disbelieve (waāllazina kafaru), and deny Our revelations, such are rightful Peoples of the Fire. They will abide therein ( 2:39 )
exact word kafir :
waaaminuu bimaa anzaltu mushaddiqan limaa maāakum walaa takuunuu awwala
kaafirin bihi walaa tasytaruu bi-aayaatii tsamanan qaliilan wa-iyyaaya faittaquuni ( 2:41 )
And believe in what I reveal, Confirming the revelation which is with you, And be not the first to
reject faith therein, Nor sell My signs for a small price, and fear me and me aloneā ( 2 : 41 )
huwa alladzii khalaqakum faminkum
kaafirun waminkum mu/minun waallaahu bimaa taāmaluuna bashiirun ( 64:2 )
It is He who has created you; and of you are some that are
unbelievers, and some that are believers; and Allah sees well all that ye doā ( 64 : 2 )
so, it doesnt mean he/she has to be a combatant, hostile nonbeliever, etc. for a non-muslim to be called kafir.