Guiding Questions:
1. How does this card stand on its own (analyze the HP, attacks, Abilities, etc. of the card)?
The card's only real purpose is to stall. With an attack for one energy, it can reduce damage done to it by 40. This is enough to survive a Blue Flare attack, and using Eviolite only further increases its survivability. However, 90 HP isn't that good for an evolving basic, and it is almost guaranteed to die as soon as it doesn't use its Reflect attack. It's other attack does 50 for three and it has a horrible lightning weakness. All in all this pokemon wouldn't be all that great before dragons, and in the current format it is very unimpressive indeed. The bar was raised with the dragons but apparently Sigilyph didn't get the memo.
2. What role does/could this card have in the metagame?
Purely as a stall engine. Use Sigilyph as a wall to set up behind. Unfortunately, Pokemon Catcher can eliminate the stall game, and unless you run basic psychic, dropping an energy will put it in blue flare KO range, Reflect or not (due to ten damage from rainbow).
3. What cards, if any, does this card combo with?
Max Potion. Take a hit. Heal it all off. Drop an energy, repreat. Eviolite to reduce damage to make the one shot almost completely impossible. Vileplume potentially to block Pokemon Catcher. The KY tech (Kingdra/Yanmega) could theoretically be something used on the Bench to put on damage while stalling, then devolve with Jirachi. This is kinda clunky however, and utterly useless against ZPST due to no evolutions, Outrage, and weakness over your stall engine.
4. Give it a rating (out of 10), and explain why you have given it that rating.
3/10. While not completely useless, it may as well be in this format. The stall engine gets rolled over by pluspower or a foolish rainbow drop, and nothing in the world will protect it from Zekrom or Magnezone. It doesn't help you set up, so while you're stalling for time, you're also going to need more time due to no refresh or draw from your starter. Considering Cleffa has the potential to stall, free retreat, and a free attack that gives you hand refresh, this card will certainly never see competitive play in our current format.