Never is there an entire episode of Rugrats that contains one but not the other, and they were among the characters who appeared in the original Rugrats pilot episode, Tommy Pickles and the Great White Thing in which Tommy is trying to get them to go exploring in the bathroom. They also appeared in all three of the Rugrats movies.
Phil and Lil are twins who are alike in every possible way. They were even dressed to match: They both wore pink-and-black striped shirts, and overalls with a handkerchief on the left side and a duck on the right, and both had a small amount of brown hair on top of their oddly-shaped heads. Phil wore blue shoes and shorts and was drawn without ear lobes. Lil wore pink shoes, a pink bow on her hair and no shorts, and was always drawn with ear lobes.
They also shared the same interest: consuming worms (which they have often called "Chocolate Spaghetti") and toilet water. They often used their "full" names, Phillip and Lillian, especially when arguing. Their parents Betty and Howard often confused the two, despite permanent differences like the ears, as well as of course their sexes.
Unlike Angelica and Chuckie, neither Phil nor Lil ever becomes potty-trained or learns to speak to adults throughout the duration of the Rugrats series. This is, in part, because they remain between 12 and 18 months of age and are treated like they're the same in every way. It would be almost inevitable that Lil would've been potty-trained sooner than Phil, so they just kept them both in diapers in order to make this difference unknown.
In All Grown Up!
Before All Grown Up!, a few Rugrats stories touched on attempts to either differentiate the twins personality-wise ("Twins Pique", 1993), or physically separate them ("Together at Last", 1992).
However, separating the twins only got serious in the new spin-off. Its premiere, "Coup DeVille", was focused on the twins. Lil strongly retaliated at being paired with Phil on a science project, as well as being treated "like a single package" with Phil, and even refused a trip to Twins Canyon with her family on that principle. This episode also repeated the "Lil is older" fact from the original series, and also revealed Lil to have an even longer name than was previously let on: Lillian Marie Jill DeVille. Lil moved to a separate bedroom in this episode.
Since then, the twins have evolved distinctly different personalities. Phil still enjoys repulsive objects (although he is beyond eating worms and insects, he once won a science fair by growing mold on cheese and socks), and has been referred to as "socially inept", while Lil is beginning to find interest in boys and become more social, and she has also picked up an interest in sports, particularly soccer.
Referencing their baby days when their favorite thing was to eat worms, Phil & Lil's favorite food in All Grown Up is Spaghetti.
In the episode Susie Goes Bad Lite, it is revealed that Phil is a good cook.
A new look
In season two of All Grown Up!, all the characters got new looks. In the second season, Phil was presented as being more "decent" when it came to apparel and his hair. In the first season, his hair was messy and always wore a t-shirt under a collared shirt with green cargo pants. Soon after, he began to sport more outfits, such as jackets and other variations of collared shirts with blue jeans. His hair was neater too. It was drastically similar to Lil's hair, only his was a bit shorter and had more of a part in the middle. Lil and Kimi both received new hairdos. Lil's hairdo was hair straightening out with one streak dyed pink (the color has varied, however). In one particular episode, Lil had Phil dress up like her but when he made her look bad, she donned a pair of pink sunglasses, long blonde wig and a wide brimmed red hat with a feather.