How can u increase your redline? (some players got positive redlines)

Your redline goes down when you fold, so you stop folding.. Instead you call/bet a lot.

How this affect your playing style? Is it directly related with WWSF?

Nah not rly, i't more complicated. If you play a really loose style preflop it's hard to have a high wwsf. Same goes with multiway pots etc, the more multiway pots the lower your redline will be. Tight pre-flop players will keep a very high wwsf because they will have such a strong range in general.

I got like 47-51 wwsf in general, 48-49 over bigger samples and I'm losing a bunch in redline in 6max NL.

Rake is a factor as well (Rake is lower when playing higher stakes -> easier to keep up redline)

You shouldn't care about redline fwiw there are players crushing the games whom are redline warriors and there a people who are crushing the games a losing a bunch in redline. I would just guess that you play lower stakes by your question and at lower stakes you will se A LOT of different styles be winning styles.

5b/j light, 4b/c wide, station the river more and bluff more...having high WWSF and redline can certainly correlate but one doesn't necessarily yield the other...playing styles that favor backdoor agression tend to yield more +redlines IMO.