Pase lo que pase

Pase lo que pase is one of those formulaic expressions where the subjunctive is used twice, separated by some kind of connecting device. Many of these correspond to English adverbs endings in -ever: whatever, however, wherever.

• Pase lo que lo pase. Whatever happens.
• Me voy a casar con Javier, pase lo que lo pase, ¿oíste? I'm going to marry Javier, no matter what. Nothing can stop me. (telenovela dialogue).
• Pase lo que pase, quiero que sepas que ... No matter what happens, I want you know that ...
• Digas lo que digas. No matter what you say. Say what you want (but it won't change my mind).

In these examples, the speaker is considering all outcomes and discarding their relevance. Other uses of no matter or whatever in English need different translations: Do whatever you want. Haz lo que quieras. Whatever! Como sea.

Sea como sea

• Sea como sea tu pareja. No matter what your partner is like. However your partner is. However your partner may be. Notice that tu pareja is the grammatical subject of sea here.

Whether or not

When you want to express the idea of 'whether or not', use a subjuntive construction with 'or'. Repeating the verb here is optional.

• Voy a estar contigo, quieras o no (quieras). I'm going to be with you, whether you want it or not. Whether or not you want me to.
• Votes o no votes, no te sientes representado. Even if you vote, you don't end up feeling represented. (a political sign)

You can get along without these expressions, but try to use them in your Spanish when the opportunity arises. You'll be expressing yourself more naturally and precisely. I've heard this type of construction called 'double subjuntive' or doble subjuntivo, since the subjuntive form is typically repeated. If that nomenclature helps you notice and remember these formulas, great.