Dr. Jekyll and Mr. Hyde ?!

I have a question to the chess-community -
starting with a little story:
There's a strange guy here. I know him and his game for years.
If I play against him, I usually win 8 of 10 games.
Sometimes he disappears and comes back after short time...
...and then - what a miracle - he suddenly plays a totally different and much better game !
A completely different style, no more mistakes and only winning!
When I tell him something is wrong, he replies that he can concentrate more or less !
But I know his character - he would never lose so many games
if he could change that simply, by more concentration !
Now I ask, is such a thing possible or not ?
I mean rather NO and above all not one moment on the other and also not the whole play style !
But if NO what could be wrong ?

to me, it seems that more than one person uses the same profile... it is not illegal (i think) but dishonest towards the other players. So, pay no mind who you are playing against ,but what kind of game are you playing.

I do not mind that he wins or that I lose. I know there is a win
or a draw or defeat in chess and I can live with it. What bothers
me is this stupid lie and that he thinks me so stupid !
First he lose sixteen times in a row and then little later he
comes back and hits people with 2000 and more points !
And he has the ass to tell me, that he has played and no other !
And expects me(us) to believe that !
I write "us" because there are others who know him well...

it could be helpful if you share some games....

@SasaTalpes I'm pretty positive that allowing someone else to use your account is against the terms of service here. Can someone confirm this? It sure should be against the rules for obvious reasons.

@WarOverABoard I think you might be right...i am not sure but for the moral stand point and common sense requires that it should be banned or illegal...

I just say that he suddenly plays quite differently and, above all, much better. What means, it is not proved that he is playing another one for himself. And a second user is very hard to prove when playing on the same PC. On the other hand, a program would certainly be easier to prove ?

In my chess club I sandbag often for the first game that I play with someone and tell them I am some 900 rated patzer that sacks everything for mate on move 1.

No I don't actually do that :P

Hugalex, if you suspect a case of multi-user account or engine assistance, I would recommend reporting the issue to the lichess staff. There's no harm in doing so.

It is the main problem of cheaters. They build their high ranks and then just there is no way out - they have to use a chess engine to keep the rank. Their real skill is so poor that it is the only way they can exist as a 'great player'. I reported two guys spotted here who after some mistakes and blunders in the openning, the rest of the game (over 30 moves) could play without any inaccuracy. No action was taken against them, they are still pretending good players. Poor kids lying to themselves.