Is there a way to escape or defend yourself from a tornado kick? I've heard there are some very fast kicks that you can hardly defend against. What should someone would do if the rival has already started such a kick? Any videos would be appreciated!
Is there a way to escape/ defend yourself from tornado kick?
Ummm... yes. If there wasn't, all martial arts would only practice tornado kicks, but in fact relatively few martial arts use the technique. Those kicks are horribly telegraphed, which means the defender have a lot of warning and time to decide what to do to defend or counter-attack. They're not especially powerful either, so can be blocked easily enough, and with very little movement from a typical guard position. If they're kicking towards your chest, an inward block with outer forearm is the most natural block. The back hand can provide further coverage. If they're kicking towards your back, an outward block with outer forearm. I'd recommend moving towards your opponent and slightly sideways - whether you move into or away from the kick, it'll move you out of the area where the power could achieve decent focus. Moving towards the kick tends to be most disruptive and the kicker has less time to bring the leg up as high. The kicker is likely to be off balance and vulnerable as they land.
I heard that there are some very fast kicks that you can hardly defend from.
True, but this isn't one of them. A low front-leg front snap kick to the opponent's front knee is an example of a kick that's hard to defend against - from a typical taekwondo fighting stance it needs virtually no telegraphing. It's also illegal under every full-contact competition rule-set I've ever heard of, in any martial art or sport. Someone with a very good low front-leg sidekick can also be hard to defend against.
Generally, the kicks that seem boringly simple are often the most useful, and these "flashy" kicks are predominantly a distraction best avoided from a martial arts / self defence perspective. Something seeming simple doesn't mean it's actually easy to do well... mastering front, side, turning/roundhouse and back kick keeps most talented, hard-working practitioners busy for a couple decades.
What should someone would do if the rival has already started such kind of kick?
Again - you have a lot of time, so unless you're out of position or their foot's already 10cm from your face, you can defend with a forearm block as above. Nothing else will be more minimal or faster.
The setup for one of these kicks is ridiculous, and I question if there is even a significant power difference between one and a well delivered roundhouse or side kick. I feel good footwork is the best defense, since anything except a stationary target is going to foil the effectiveness of execution. I'm a big, aggressive fighter, and I always step inside when someone tries this kind of nonsense. You might get hit with a suboptimal part of their leg, but if you press the attack they won't have an opportunity to recover. If you time it right you can strike/snatch them out of the air and they have no base with which to prevent you from dropping them on their head. You can always step out, but then you might give them time to recover.