The words loose, lose, looser, and loser often cause problems with both native and non-native speakers. The reason is quite obvious: their spelling and pronunciation are very similar although not identical.
The words lose and loser are related in terms of their meaning, spelling (one ‘o’ only) and pronunciation, but have nothing to do with the words loose and looser (see below). The meanings of the verb ‘to lose’ include the act of no longer having something or someone (to lose a friend, to lose your keys; German: verlieren), or of being defeated (to lose a game). The corresponding simple past of ‘to lose’ is ‘lost‘, and the past participle is also ‘lost’. Nouns related to the verb ‘to lose’ are loss and loser (someone or something who/which loses something, such as a battle, game or election). You can find more meanings here. The pronunciation of lose is [luːz], listen here. The important aspect is the pronunciation of ‘se’ as ‘z’.
The word ‘looser‘ is an adjective, namely the comparative of ‘loose‘ (the superlative is loosest). Therefore, you can and should not call anyone a ‘looser’, because (a) that noun does not exist, and (b) if it did exist, it would have a very different meaning! One reason why this mistake is so common is that a spell checker will not complain about ‘looser’ (although a grammar check might reveal the mistake). On the other hand, ‘loose’ exists both as a noun (to be on the loose) and as an adjective (for example, the opposite of tight, exact or strict; German: locker). The pronunciation is [luːs], listen here. Hence, it is the pronunciation of the ‘se’ as ‘s’ that distinguishes loose from lose, although this subtle difference may not be easy to learn and recognize for non-native speakers.