I missed this question when you first posted it, so I don't know if you are still looking for answer, but I am also focusing a lot on ink lately so I relate to your question. It's really not specific to ink though, and not necessarily about line weight; it's really about value in your overall picture. The moose is clearly distinguished because he's a different overall value from the white background. The owl isn't distinct because all the business in the background is roughly the same value as the owl. To make the owl more visible, the foliage should have a different value read - i.e. if you squint your eyes and look at overall patterns of light and dark, the owl shape should be distinct from the background shape. Whereas adding value in graphite or charcoal is easy by just lightly or firmly shading things, with straight ink is tricker, so normally this is where hatching comes in. Lots of parallel straight or curved lines can add value, and just as with graphite drawing that value can represent shadow, or the object's intrinsic color. So if, say, the owl is light gray and most of the foliage is deep green, and the branch is a darker-barked tree, use hatching on the foliage and tree to darken it. You could also use light straight-line hatching on the sky to give it a light-mid value. Some great artists to check out for hatching technique to produce value are Franklin Booth and Albrecht Dürer, although most good inkers today (in graphic novels etc.) apply these techniques. One other approach you can take if you work with dip pens is to use diluted ink to make the actual lines lighter on some elements. With this approach you can also use a wash of diluted ink with a brush to give a solid tone to an element - either an object or the background or the shadows etc. Search for "Ink wash painting" online to see examples of this approach.

Hey! Your moose and owl look super cool. There are a few ways to make a character pop from the background. 1. thicker outline using ink around your character 2. create contrast. Either have a lot of detail on the owl or barely any detail on the owl with a heavily detailed background. (ex. your moose stands out because nothing is in the bg; it's white.) 3. See what other inkers are doing! Mark Morales is awesome: https://youtu.be/862pfBBB3cQ?si=NAD66tnGcHAii9Na

These are gorgeous drawings, @Judah Rojas. There a lot of different ways you can separate an object from the background -- line weight is one of them for sure. However, I really like that you used more a more dense amount of lines in the moose to make it appear darker and less lines in the lighter areas and the botanicals. That makes a really nice separation while still maintaining the same line weight. If you want to experiment with line weight, you could try using a thicker line weight around the contour of the owl. Use a spectrum of line thicknesses to avoid a coloring book effect. Think of the lines like shapes and design them to follow the form. You could also try adding more darker shapes in the background, in the form of thicker lines. You could place them where shadows might fall, keeping the parts in light at a thinner line weight. Personally I like that your lines are the same thickness. Playing with the density of your lines in different areas could be an interesting approach to explore as well. I hope this gave you some ideas. Good luck drawing :)