Hi Gerd, let's see if that changes things a little then! Betis has always been the underdog, and I know your love for underdogs!
In Spain, Betis is a funny team with funny supporters and a very special personality. Out of Sevilla fans, everybody loves Betis.
It comes from long ago. Sevilla always was the club of the big lords and rich people, while Betis had always been the team of people of the street. Sevilla always had more money, so won more than lost against Betis. That forged a special way of living for the Betis club.
"Viva el Betis manque pierda!" it's a famous spanish phrase (Hurray for Betis even if it looses) and resumes the attittude of supporters: have fun even in defeat, joke about it but have the team at your heart whenever you go.
Betis supporters do even create spanish Betis nicknames for the players. When a guy with a complicated balcanic name arrived to the club (something like Belocidacevic, I can't remember), the supporters decided to simply call him "Pepe". So he was playing at the Benito Villamarín and people chanting "Peeeeeepeeeeee".
They're full of that kind of stories and that's one of the reasons a lot of people is keen on Betis more than to Sevilla, that has historically been a very tough team.